I see a lot of people saying this is due to lack of competition. I hate to break this to you but it isn't that. A lot of European countries thinking the competition will drive the costs down, including on the supply side, and liberalizing the market realized not long after that this did nothing to reduce the cost. More often than not it drove the cost up.
The problem is that the public transportation is never truly free market, as they are always heavily subsidized. More companies relying on subsidies to do business doesn't change the fact. On the supply side, bus manufacturers have the same. US federal govrhas strict requirements to buy American made busses. I think NAFTA might be ok too, but not sure. In any case, what the US government paying for is manufacturing jobs and this is not necessarily a bad thing. Or let's put it another way. Those busses can be produced in China or Japan for much cheaper. But then you will let go of this industry, and have more dead towns and small cities without jobs.