That pretty much leaves one option: residents of suburbs themselves don't like the idea of mixed-use development and they let their planning boards know it in no uncertain terms. What's more, since developers don't seem to be fighting residents very hard about this, I have to assume they're skeptical that they could rent out all the space. If they really thought they could make a buck off developments like this, they'd be bribing city councilmen left and right.
In other words, I suspect that just because people visit Downtown Disney on their vacations, it doesn't mean they're pining away for small town life. They aren't pining away for roller coasters in their backyards, either. In the end, some people like cities and some people like suburbs, and it's just a matter of taste. The people who like cities whine about gentrification and white flight, and the people who like suburbs whine about anything that increases noise or traffic congestion. Both sides seem pretty dedicated to keeping their own patches of land just the way they are.
I'm sure there's a lot of truth to this, though I don't think it's a complete explanation. I do think that developers get locked into a mold and other governmental/legal/institutional issues are at play, but nonetheless this is certainly a nontrivial part of the story.
I used to live where Kevin does, in the kingdom of Irvine (a place I could talk about ad nauseum if I thought anyone cared). While I was there I was told an anecdote. I'm a bit fuzzy on the details, so don't take any of the precise facts as gospel, but the basic gist is true.
Larry Agran, who has served as mayor a couple of times, had at some point pushed to put an urban corridor some place in the city. From what I remember this wasn't even a serious mixed-used multi-block area, but instead a few blocks on one street which would have street level commercial/retail and perhaps some apartment blocks on either end. Artist renditions of this were printed up, and the opponents of this plan (which, it must be made clear, would've involved an infintesimal portion of the land of Irvine) took these renditions and for their ad campaign added in pictures of panhandlers, the homeless, etc... The point was that anything even slightly resembling a "city," even a tiny version, was going to attract The Wrong Types.