If the government values and recognizes the savings in preventative medicine, they can reward those delivering it. (Even if it wasn't in a single payer system, there are other ways the government could do that, such as stipends, assistance with educational debt for those who choose and stay as PCPs).
Problem is politicians only ever look at costs in the sense of "what is this costing now?" They look at how much it costs to deliver polio vaccines, without appreciating how much they'll save down the road by spending that money now. (That is not unique to US politicians, it's the same everywhere of course)
Americans are by and large fat lazy slobs. Preventative medicine is more or less free for anyone who wants it. Americans are unwilling to make lifestyle changes that will lead to better health. It sucks, but that's the way it is. Unless we figure out some way to force exercise and healthy eating habits, all the preventative medicine visits in the wold won't make a dent.
Edit: I may have used to broad of brush. A large percentage, probably half of Americans are fat and lazy. They don't care enough about their own health to put down the twinkie and big mac yet think for some reason the rest of society should care about their health when the consequences of their lifestyle arrive.