Why is there no need for any further revelation?

From a Christian point of view:

1 Corinthians 13:9-10 "For we have partial knowledge and we prophesy partially, but when what is complete comes, what is partial will be done away with."

Many feel the "what is complete" is talking about God's written Word, the complete Bible. Since we now have the complete Bible (When Paul wrote this, most of the New Testament had not been written yet), we know what we need to do now and what is coming in the future.

As far as future revelations, the logical conclusion would be there would be more instruction once God's Kingdom rules over the Earth as it is in Heaven.

The Bible also says that scrolls would be opened during the future resurrection at Revelation 20:12-13 - "And I saw the dead, the great and the small, standing before the throne, and scrolls were opened. But another scroll was opened; it is the scroll of life. The dead were judged out of those things written in the scrolls according to their deeds. And the sea gave up the dead in it, and death and the Grave gave up the dead in them, and they were judged individually according to their deeds."

Whether those are literal scrolls, or a "new Bible" so to speak, we do not know for sure.