I can't speak for everyone else, but here's why I don't play MN, despite very much wanting to:
Patch hell: keeping straight what patches are necessary, how to install them, and in what order was a hassle. I had the right combination of patches at one point, but now it seems I don't, and I can't see any servers, not even the empty ones I used to. Perhaps I'll try the ro.pl installer.
Having to start either RO or MN: it's not a big deal, but if I have to choose between definitely playing on a full RO or CC server, or spending a few minutes to maybe find a MN server, I'll take the former.
Now, as others have said earlier in the thread, these aren't major, insurmountable issues. That's definitely true, if each is considered individually. But if a few factors each make it a little harder to play MN, taken together, it's possible to make it too much work for a critical mass of people to play at any given moment.
It's too bad. The new maps and weapons look really awesome. Clearly a lot of good work went into making them; it's a shame more people don't get to experience them.