Could it simply be a case of the MSX scene being much bigger?
I'm not sure about the Amiga community size, but MSX scene is not that big. Most of those games are made by a small European group (Netherlands and Spain mostly). I should say no more than 20 ppl, divided into coders, musicians and gfx artists.
My thoughts are:
1) Developing a game on Amiga is much harder than it is on MSX
2) There is a lot of other things you can do with an Amiga than you can`t do with an MSX. (i.e. WEB, 3d modeling and etc...)
3) I think, that mostly due to complexity, emulators for MSX are far better than WinUAE and others. They are simple and filled with hacking tools like debuggers, rewind feature and several others.
4) About ppl talking about making money out of a hobbyist platform, at the msx scene, all of the games are freely available to download. Some guys who tried to sell .ROM files were deeply criticized by the community. Normally the best games receive reals versions on cartridge with fancy labels, manuals and box. Those are sold, and everyone happily pays for them.
just my two cents.