Although I could only pick one, I agree with 3 of those reasons.
1.) Sega didn't care about the games and never advertised them much or even at all.
2.) Even with the proper advertising, the games were pretty meh. I may have nostalgia for TCon, but I can't deny its many bugs and gameplay problems. Conduit 2 had a bunch of these as well. However, they did have some neat ideas and weapons. I could go on about this, but I won't, lol.
3.) It's on the Wii. That pretty much throws off a lot of "hardcore gamers" off who think that because it's on the Wii, it's gonna suck. Well, to be honest, though, Even if that wasn't the case, the games were mediocre, so they weren't really missing much anyway...
I wouldn't want to fall into the trap and say it isn't as popular as AAA games because it isn't one, but that's kind of why. Both games lack polish, Conduit 2 had next to no marketing, both limited by the Wii, and have no support with updates or DLC. And there are more people looking for that than not.