There are very real economic reasons to build out rather than up, when you have suitable land nearby. Especially in areas like Colorado where land is cheap and regulation is light, building nice big cheap (in terms of construction costs) single family homes sounds pretty good, vs. building expensive (per square foot of usable space) high-rise condos. Construction costs are lower, and you end up with a bunch more living space, a garage, etc, etc. And from a buyer's perspective, you have a lot more control over a single family home.
And yeah; you can have public transit in the 'burbs? but it's not going to be popular. Because you have so much more living space than a city dweller, you are likely to get more utility out of the hauling capacity of a car. And having that extra living space also dramatically lowers the cost of owning the car; You don't have to pay for parking, and you can do basic work in the garage. (you can save a giant wad of cash doing basic stuff like swapping rotors yourself.)
Personally, I think the economic realities of the suburbs mean that only the poorest of the poor are going to not own personal vehicles; because of this, public transit is built for and associated with the poor in all but the most built-up parts of America. "Domestic help" is not fashionable here, either, so for the middle class, there isn't a lot of upside to letting poor people into their neighbourhoods. (I'm wondering if this is going to change with the rise of in-home elder-care. There certainly are plenty of Americans willing to do that sort of work for wages the middle-class could pay.)