Actually, I think this tells us more about editorial decisions and market reality.
Peter David does have a few flaws where marketability is concerned, but that is not really the issue; these days, 80 issues on any title are a memorable run and worth of applause. That is more than Marvel managed to get on Gruenwald's Quasar or Thunderbolts (which _should_ have been cancelled rather than shamelessly 'reinvented'

. If anything, he is an incredibly high market success.
Sadly, that is no longer a determining factor in a book's viability. The market for comic books has shrank so much and the number of titles increased to the point where there are possibly more people commenting books here on Usenet than actually reading them.
Part of the reason why the 'Priest curse' seems to be rubbing off on Peter David is that they are, in fact, similar writers on some respects and face some of the same challenges - they write superb stories that tend to rely a bit too much on past continuity and at times become inaccessible. If the book you are writing has a title that begins with the letter X, that is a virtue of sorts. If not, you have to be very good indeed to succeed, because you are effectivelly writing mostly for current and past readers, running the risk of alienating a few of them on every major change, and basically praying that word of mouth becomes good enough for your book to be chosen over about 200 others by those willing to try a new book.
The end result is that their fates are strongly correlated to the welfare of the industry as a whole; there is however a stronger inertia to their books than to most, because the end product is so reliable and above average. People who dig Priest's and David's books usually really dig them, and will follow them from one book to another. But even that is not always enough in todays's market.
Best, Luis.