Because WP7 is out (and struggling) meanwhile MeeGo is struggling to be released in a final, fully usable form that can be a mass-marketed OS, something that never was targeted nor obtainable with Maemo.

There are many other reasons; however it's easier to just look at the bottom reasons of that one is out, somewhat liked and the other, a ton of money invested, nothing final as of yet. Outside of this community, it just doesn't make sense to have nothing to show for your chosen OS of your future highend phones.

Thus... the question of "Why not use WP7, Nokia? It's there, it's square... get used to us thinking we want it there!"

Or something like that.