Response
The Doctrine and Covenants contains foundation revelations establishing core doctrines, the organization of the Church and the priesthood, and the manner of ordinances. Prophets after Joseph Smith have and continue to receive revelation, but only in a few circumstances have these revelations been foundational enough to necessitate them being added to the LDS canon.
This pattern is reflected in the Bible as well. Moses, the founding prophet of that dispensation, produced five books of scripture, which were the basis of religious instruction for the next several centuries. The books that followed Moses for the next couple centuries were mostly historical accounts (Joshua, Judges, Ruth), with only occasional revelations recorded by subsequent prophets.
Joseph himself stopped receiving so many revelations as the government of the church was established. At this point in his life he said that the Lord should not be petitioned for every little thing, especially if revelations on the same subject had already been given or information "about the things the knowledge of which men ought to obtain in all sincerity before God for themselves." After the 12 apostles were organized, the number of revelations that Joseph received decreased dramatically. Around 1835, nine years before his martyrdom, Joseph stopped receiving revelations as people remember him to have done previously.
Revelation continues in the Church
Elder John A. Widtsoe explained that we still have revelation in the Church, even if it is not being added to the Doctrine and Covenants:
There is...need of continuous revelation. However, we must understand that there are two classes of revelation given by God to man. The first deals with the structure and content of the plan of salvation. Once given it does not need to be given again. Adam received it. *** Christ gave the same revelation to man in His dispensation. So did Joseph Smith in his dispensation. The foundation, or platform, once given does not need to be given again unless men forget the truth.
Then there are revelations that fit the changes in our lives, meet our new needs, help us overcome unforeseen conditions—revelations for our daily guidance.
This great country, the United States of America , has found itself in a great depression. We have the Gospel. What did the Lord do? He spoke to his Prophet, and we have what is known as the Welfare Program. It is the application of the eternal principles of the Gospel to present day needs. It is as revelation. We have that type of revelation continuously.
So, when people say: "We ought to have revelation now as we did in the day of Joseph," we must answer, "Open your eyes; we do have revelation every day; such as we need from day to day." The examples of the revelatory experiences of the prophets from Joseph Smith's day to now are too numerous to list here. The full wiki article gives examples of revelations from prophets through the years with all references and sources.
http://en.fairmormon.org/Revelation_after_Joseph_Smith
Revelation in Contemporary Times
More recently, Elder Dallin H. Oaks, of the Council of the Twelve said, “ Why don't our talks in general conference and local meetings say more about the miracles we have seen? Most of the miracles we experience are not to be shared. Consistent with the teachings of the scriptures, we hold them sacred and share them only when the Spirit prompts us to do so…In bearing testimonies and in our public addresses we rarely mention our most miraculous experiences, and we rarely rely on signs that the gospel is true. We usually just affirm our testimony of the truthfulness of the restored gospel and give few details on how we obtained it. Why is this? “Signs follow those that believe.” Seeking a miracle to convert someone is improper sign seeking. By the same token, it is usually inappropriate to recite miraculous circumstances to a general audience that includes people with very different levels of spiritual maturity. To a general audience, miracles will be faith-reinforcing for some but an inappropriate sign for others.
“Visions do happen. Voices are heard from beyond the veil. I know this. But these experiences are exceptional. And those who have these great and exceptional experiences rarely speak of them publicly because we are instructed not to do so (see D&C 63:64) and because we understand that the channels of revelation will be closed if we show these things before the world.”
Elder Boyd K. Packer said, “ There has come, these last several years, a succession of announcements that show our day to be a day of intense revelation, equaled, perhaps, only in those days of beginning, 150 years ago But then, as now, the world did not believe. They say that ordinary men are not inspired; that there are no prophets, no apostles; that angels do not minister unto men—not to ordinary men. That doubt and disbelief have not changed. But now, as then, their disbelief cannot change the truth. We lay no claim to being Apostles of the world—but of the Lord Jesus Christ.
“The test is not whether men will believe, but whether the Lord has called us—and of that there is no doubt. We do not talk of those sacred interviews that qualify the servants of the Lord to bear a special witness of Him, for we have been commanded not to do so. But we are free, indeed, we are obliged, to bear that special witness…Like all of my Brethren, I too come from among the ordinary people of the Church. I am the seventy–eighth man to be accepted by ordination into the Quorum of the Twelve Apostles in this dispensation. Compared to the others who have been called, I am nowhere near their equal, save it be, perhaps, in the certainty of the witness we share. I feel compelled, on this 150th anniversary of the Church, to certify to you that I know that the day of miracles has not ceased. I know that angels minister unto men. I am a witness to the truth that Jesus is the Christ, the Son of God, the Only Begotten of the Father; that He has a body of flesh and bone; that He knows those who are His servants here and that He is known of them. I know that He directs this Church now, as He established it then, through a prophet of God. In the name of Jesus Christ, amen.
1978 Revelation
Many witnesses described the 1978 revelation on the priesthood. Wrote the past LDS Church Historian:
”As a historian I sought to learn the particulars and record them in my private diary. The following account is based on dozens of interviews with persons who talked with church officials after the revelation was announced. Although members of the Twelve and the First Presidency with whom I sought interviews felt they should not elaborate on what happened, I learned details from family members and friends to whom they had made comments. . . .
“Those in attendance said that as [President Kimball] began his earnest prayer, they suddenly realized that it was not Kimball's prayer, but the Lord speaking through him. A revelation was being declared. Kimball himself realized that the words were not his but the Lord's. During that prayer some of the Twelve -- at least two who have said so publicly -- were transported into a celestial atmosphere, saw a divine presence and the figures of former presidents of the church (portraits of whom were hanging on the walls around them) smiling to indicate their approval and sanction. . . .
“At the end of the heavenly manifestation Kimball, weeping for joy, confronted the church members, many of them also sobbing, and asked if they sustained this heavenly instruction. Embracing, all nodded vigorously and jubilantly their sanction. There had been a startling and commanding revelation from God -- an ineffable experience.
“Two of the apostles present described the experience as a "day of Pentecost" similar to the one in the Kirtland Temple on April 6, 1836, the day of its dedication. They saw a heavenly personage and heard heavenly music. To the temple-clothed members, the gathering, incredible and without compare, was the greatest single event of their lives. Those I talked with wept as they spoke of it. All were certain they had witnessed a revelation from God.”
Proclamation on the Family