The biggest issue is the investment money to build the infrastructure, housing, etc. And of course, crucially, business draw to live there.
I suppose if Silicon Valley types like YCombinator who invest the money, and Google (Larry Page regularly talks about Google building a city) who is a large industry were to go forth on one of these model city type projects, the initial draw would be those exact things. If Google wanted a model city to work, Google should ensure that Google is the first business to move a large portion of it's workforce there with major incentives for people to do so. People serving that initial seed workforce other services would follow.
A new city would be founded much the way they did in the old days: Company towns.
Aside from the environmental issues, and the fact that not too many people would be crazy about living there -- what about the generations-old farm families who just... don't want to sell? At any price?
Which is what I meant about there being few places of significant scale (aside from protected wilderness areas that ought to remain as such) that are genuinely "empty."
Which is precisely my point -- you might want to look at how the regimes like those in charge of Kazakhstan and China operate. In particular, as to how they "leverage" their populations complying with these grand futuristic visions.
Not to mention the... ironic outcomes of some of these spectacular initiatives:
