I've long suspected this...

In yet another internet hideyhole, I've found an interesting 2006 interview with Samuel R. Delany (SF legend, literary fictionist, critic, pornographer, theorist, wild man, teacher, and all-around Dude Who Looks Like Santa), in which the Big Man has this to say about the contemporary publishing industry in the wake of its mergermaniacal 'contraction' (a contraction from which nothing of non-monetary value will be born):

Commercial publishers today are far more distrustful of good writing than they have ever been before, and usually won't consider it unless it comes with some sort of ready-made reputation or gimmick. In the last half dozen years, writers have shown me rejection letters from publishers such as Harcourt Brace that actually say, under the letterhead, "We're sorry. This book is too well written for us." This means that competition is of an entirely different order than it was, say, thirty years ago, when such a letter simply would not have been written.

Thanks to interviewer Josh Lukin and to The Minnesota Review for publishing this interview on their website.