Because like any fans of anything they are fickle beyond fickle. I've seen people who have been fans of a show for years then one bad episode and they are ready to jump ship. I swear, those people would make great network executives.
Yeah, many Trekkies are like that too. And I think it was David Tennant who joked during an interview that one of the first questions he was asked when he took the part is "when are you leaving?"
Moffat has been great for the show, RTD made the general public care about the show (which annoyed some people who preferred having Doctor Who their own private niche fandom). And anyone who thinks they've abandoned the original series hasn't watched the show otherwise they'd have noticed Sarah Jane, the steeped-in-history Doctor's Wife, and maybe they were so disgusted they didn't notice Graeme Harper's directoral credit on a number of key episodes.
Back on the original point, no I don't want to see O'Bannon as showrunner, but I wouldn't mind seeing him write an episode if the BBC were to ever open the door to non-British writers (that's not quite fair; it's more likely the writer's unions that wouldn't allow that). A more likely scenario (start writing those letters) is to see IDW and Boom! collaborate on a Farscape/Doctor Who crossover special in the comics. (Keith DeCandido has written for both Farscape and the Doctor Who franchise - short stories in the latter - so that could work.)
My assumption is Gatiss for next showrunner, with Whithouse a possibility if Being Human is finished by then. Outside odds though are favoring Neil Gaiman based on The Doctor's Wife; hey, a year or so ago there were people calling it a myth that he'd ever write for Doctor Who because they couldn't afford him!
But I think it'll be Gatiss because - criticize all you want - they have so far been picking showrunners with longstanding connections with the series: RTD was a lifelong fan who wrote a Virgin New Adventures novel; Moffat likewise except he wrote short stories for Decalog as well as the well-received Curse of Fatal Death spoof in the 90s; Gatiss not only is a Doctor Who novelist of longstanding, but he was also (alongside Nicholas Briggs, Bill Baggs and Keith Barnfather) one of the producers/writers/directors who kept Doctor Who-related productions alive during the 1990s through his involvement with spin-offs such as PROBE.
PS. Speaking of Farscape, just found out on the Sarah Jane Adventures Series 5 thread that the villain in the season premiere was played by the same actress who played Teeg in Farscape (she was the blonde officer who served under Crais in the first season and got her neck broken for the effort).
Alex