Er...because he said so?
see http://en.fairmormon.org/Joseph_Smith/N ... rch_intact
"I am the only man that has ever been able to keep a whole church together since the days of Adam. ... Neither Paul, John, Peter, nor Jesus ever did it. I boast that no man ever did such a work as I. The followers of Jesus ran away from Him; but the Latter-day Saints never ran away from me yet."
As first explanation, fairwiki said:
Public and private statements by Joseph Smith indicate that he was in fact a humble servant of God, and this particular statement should be interpreted in light of that fact.
If this is not enough, fairwiki adds:
Entries made by scribes, not Joseph... Even in the History of the Church (where the speech is recreated in 6:408-409), it is described as resting upon a "synopsis" by Thomas Bullock. Is it, therefore, a primary source? Arguably not.
If one is not convinced yet, fairwiki has another argument:
Is the History of the Church unreliable, then? It worth saying that in the general reliability of the History of the Church, in view of the way it was put together, it is not the overall thrust or narrative that is likely to be inaccurate, but the nuances, the tone, the details.
And the last trump:
1.This wiki article was originally based on a personal message from Daniel C. Peterson.