I think it is a reaction to the "Roddenberry is god" views popular in the '70s. People like to tear down those with over-the-top reputations (e.g. Walt Disney). It is significant though, that while Gene's public reputation began to crack in the late 1980s after the problems at the beginning of TNG production, people didn't start piling it on until he was already dead and unable to defend himself (Joel Engel's book being the obvious example). A few months ago I read Susan Sackett's 2002 Inside Trek
memoir, and while she obviously loved the man (whom she could never have for herself) he doesn't really come off all that favorably there either.
Someone should make use of the Trek archives at UCLA and elsewhere to write an "Inside Star Trek" series (in print or online) focusing on the individual episodes (scripts and memos) and how they developed the way they did, as David Gerrold did for his "The Trouble With Tribbles". That way the contributions of GR, Coon, Fontana, Justaman et al can be discerned clearly.