Because U.S. publishing has become a winner-take-all (or winner-take-most-of-it) game in a way it didn't used to be. Everyone's chasing big, fast hits instead of investing in authors who may gain strong followers over time. And publishers seem to think the best way to get a big, fast hit is to sign someone who's been a hit elsewhere (or with a past book): i.e., who has an audience.

That pattern isn't unique to publishing--it's happening in Hollywood, too. But the pressure can really hurt authors.