Simon could not achieve the popularity he'd have liked with his mysteries partly because he could not give in to readers' expectations of blood, gore, sex, and violence. He had too much artistic integrity.
But now as a blogger he's become what he steadily resisted being as a novelist.
In the same post, Lance also provides this wonderful paragraph:
The other problem Simon has had in breaking into the top tier is taste---he has too much of it. Tact, too. He can't be vulgar enough. He holds back on the kinky sex and graphic violence. But he's not writing Agatha Christie style Murder Among Nice, Staid Dull People Living in Cozy Little Villages Where Just the Mention of Sex Causes Grown Men to Blush and Women to Feel a Little Faint Who Done Its. He's writing Tough Guy Working the Mean Streets and Getting Laid A Lot When They're Not Getting Beaten to A Pulp pulp style murder mysteries/thrillers. There are rules, and Simon doesn't follow them.
Full disclosure: I've never read a Roger L. Simon novel, and I only read his blog when some other blogger that I do read links him, and that's not that often. I just found this post pretty well constructed.
(And it occurs to me that if you just change a few of Lance's phrasings around to alter the genres from mysteries to Science Fiction, it could be about Orson Scott Card. But I suspect that Card's a better SF writer than Simon is a mystery writer.)