Eh. I don't think it's about genre, but the tone of what you've done before. In the case of RTD, I feel like everything in the man's career was leading towards Doctor Who - the kid's focus on Why Don't You in 1985, the 6-part Buffyesque teen sci-fi drama of Dark Fiction in 1990, the three episodes of Chucklevision he did, the supernatural/horror mix of Century Falls, designed for young adults, his character work on Revelations, Coronation Street & The Grand in the mid 90s, and then the string of award-getting adult drama after that in the form of Queer as Folk, Bob & Rose, Mine All Mine, The Second Coming and Casanova, all of which added up to give him top-notch budget wrangling, production and adult TV chops.
His career is like blueprint for creating somebody to be in charge of Doctor Who; a behemoth of kids TV, soap experience for character development, and a keen interest in sci-fi and the supernatural. He was also one of that elite group of 10-or-so writers whose pub-based antics in the 90s essentially kept Doctor Who alive - of which Gatiss and Moffat were also a key part, alongside people like Rob Sherman, Lawrence Miles, etc.
Moffat didn't - doesn't - have half of the experience RTD has, which is probably why the show has in a production sense been a more creaky ship under him. However, he was chosen for the role because of his work on Who post-2005 more than anything else, which is fine.
Iannuchi is a vastly experienced TV writer and producer, but really only in the comedy and documentary formats. In that sense he'd have as much a chance of falling down on production as Moffat has or as RTD did in series 1, because Who is a different beast entirely. So I'm not sure I buy the he could produce the show better claim, really. I know he likes sci-fi from interviews, but he's never written anything in the genre, so to hand him control of the longest running sci-fi show on TV might seem just a touch mental.
Last, he's famous enough that he can do whatever he wants, and while he's spoken in interviews of admiration for Doctor Who and liking it as a child, he evidently isn't the type of mega-fan who'd turn down a massive blank cheque like RTD or Hollywood opportunities like Moffat in order to do the show. If he was, I'm sure he'd have been in that pub with the rest of them (mostly comedy writers, let's not forget!) in the 90s.