The PSP doesn't have a small userbase, there's millions of users. Haven't you seen the sales charts? The PSP is the most successful portable gaming system ever due to the fact that the release of the PSP Slim hit a record high! There's more PSP's out there than anything else because it is a portable Playstation 2.
Maybe, but there are a lot more than just millions of PC's that can run TMU.
When a game is on a smaller system, it requires less. The PSP is extraordinarily compressed; Test Drive Unlimited has 70 songs, most of which are 6 minutes long, 40x40 mile free roam, about 50 cars and multiplayer. And guess what? Atari compressed it into a UMD with space to spare. It used up 1.70 gigs out of 1.80 available on the UMD. If Test Drive Unlimited can do it, Trackmania can! I'm not saying United has to be on there, but they could worsen the graphics a bit, as they don't have to be as impressive. Do you have a PSP by the way?
I'm sure they can compress it but that's completely irrelevant, HD space is not all there is to running a game. As I posted above, the PSP is way weaker than the minimum specs of TMU. TDU has a huge developer company and lots of money behind them with which it would be no problem to rewrite the whole game to work on the PSP, but the 10 people at Nadeo probably have other priorities.
TMU would also have to be changed to use PSP's operating system which is probably not Windows with DirectX but something different. A big company like Atari might have the resources and knowledge to do so, but I doubt Nadeo does.