Ooooooooh look this thread was revived really. I'd like to present the opinions given to me by the kids who I know who love the Zero series and were introduced to MM through it.
Firstly, the art style. Typical reaction to the classic/X series art style by a typical female: 'Haha wow that looks old.' Typical reaction to the Zero/ZX series art style by a typical female: 'Awwww, that's sooooo cuuuute! It looks really pretty!'
...Okay so not the best thing to put killer robots into but still, the more modern art style appealed to the younger generation
Secondly, the GBA. Seriously, a bunch of people just picked it up because hey it was there and they could play it whenever they felt like. Pretty much everyone had a GBA so the game wasn't out of their way in any way. Anyway, BN was on it. (According to one fan that was his justification anyway.)
The story. Well actually even I was surprised by the story. You know, the fact that the plot actually mattered and all. It was quite compelling and the players liked the setting, the characters, etc. Especially Zero. The kids I talk to who started out with the Zero series are huge Zero fanboys. I guess he was just too badass eh? =w=a
Gameplay~ Controls, animation were smooth and solid. The game wasn't too easy so the 'hardcore gamers' didn't write it off as a kid's game and kept playing. Also the animation. But that's mainly just me. I love good sprite animation
Ooooh and the stages weren't like, super repetitive and uncreative or something. They pretended they were story points and stuffs as well. Fun!
But mostly they seem to talk about (according to them) the awesome characters and story. Especially Zero. And his coolness and how much they liked playing Zero because he was cool. Anyway one of them thinks that Zero was not cool in MMX at all.