Combination of good lyricism. "purist" credentials, and mainstream visibility/popularity.
When you throw in the visibility/popularity factor into the mix, the level drops significantly. When it comes to mainstream, probably. When it comes to hiphop as a whole? Not so much. But it says alot about the state of mainstream hiphop when these two are at the top with virtually no competition. But I'm gonna need to see them do more to even take a sniff at Nas and Jay.