However, I'll have to mirror what Nayoril said. There's just too many. A number of people just can't keep up with that high a number.
And going to Nayoril's second point, the amount of idols is difficult to fully develop all of them. Moreso in CG than ML and SM since contrary to popular belief, the latter two (especially ML) have story commus that give more insight on the characters. However, of course it's not nearly as much as 765Pro, which to be fair have had 10 years worth of development, games, and overall existence to be fleshed out while these spin-off characters have hardly had half that time. There's great potential, but these characters need to be in a console game for that potential to be realized by a larger number of people.
Metalpredat0r pretty much nailed it. (Although, this topic has been discussed a lot already).
Those are the weaknesses behind the spin-offs. The 765pro idols are so well established and developed since they have a long history with console games, communications, story, songs, animes, mangas, radio, drama, etc. Plus, the 765pro idols had lots of interactions between each other and their different producers throughout their history.
And, they were able to pull all of this off easily since it is just 13 idols instead of a huge group of idols (it started as 10 in im@s 1 arcade. Then, it was 11 with im@s 1 xbox360 with Miki. Then, it was 13 with im@s 2 with Takane and Hibiki although they were introduced as the main rival unit back in SP).
However, the spin-offs don’t have nearly as much history of development. Part of the reason is because they’re relatively new. But, the biggest reason (like mentioned before in the above posts) is because it’s difficult since the spin-offs have too many idols. But, like Metalpredat0r said, there is some development like mobile games, mangas, songs, etc. But, a lot more work needs to be done.
People have gotten so attached to 765Pro for the longest time, they can't see themselves be bothered to get attached to the spin-offs. Whether it be the large number of idols, the lack of development in comparison, the lack of a console game, not having their favorite idol, etc., some people can't be bothered to leave what they're most familiar and comfortable with.
I'll admit I'm bias (and attached) towards 765pro because when I first started getting into im@s, there were no spin-offs like CG or ML. (There was Dearly Stars but let’s keep it simple). All you had were the 13 idols and that was it. Like I said before they are very established/developed and have a long history of interaction between each other and their different producers. Plus, my most favorite idol of all is from 765pro.
But, I’ll admit there are some idols in ML who have pretty good artwork.
Ironically, I’m also a little bias towards Side M because of Jupiter and the other cool male idols since I can relate to cool guys.
But, above all, one should like whatever they want regardless of what other people say. Everyone has different tastes.