Something that I perceived in Ari's posts in his blog is that he didn't mind for Maemo/MeeGo having "rough corners"; that in fact he liked that.

Considering that MeeGo is supposed to be the perfect all over contender in the high end against other devices, probably this paradigm of his wasn't the best for the future, hence his departure.

Mind you, it's pure speculation on my part.

But one thing is for sure, like Quim said, he made Maemo from zero to hero. So, respect to that.