Ay-yup! We always knew that most of them weren't serious gamers...their spending habits are pretty informative.

Not surprising, really. Mobile games tend to be simple and very casual when compared to console and some PC games.
Battery life is probably a factor, but the trend for mobile device games seems to be to make 'coffee break' games, which require very little time investment or thought - you can play them absentmindedly while in a waiting room or on a break at work, and just as quickly stop what you were doing without losing anything from the experience.
compared to console games, and a lot of PC games, where having to stop abruptly after only 5 - 10minutes can ruin the tension of a scene, or being in a crowded waiting room can disrupt your focus making you lose track of plot details or just play poorly and get stuck. In some games, especially older games (becoming less common nowadays), the sudden interruption could even make you lose all the progress you made in a session, reverting back to a save point from a previous session.

I'm still playing MMR at the moment (and occationally talking to team MMR) and while its a nice game, its definitely not a true Monster Rancher game.
For me, it can only be done on true consoles. I don't even like it as much on the DS.
I can only hope another true successor will appear someday.

MMR players, keep checking here, I will be announcing a contest in about a week.