I'm sure Android on its own, can be pretty open too, but maybe they can lock it down that way for particular phones/carriers, while Nokia will give the user root for Maemo 5 out of the box?

One reason Android is growing more than Maemo at the moment is probably that there are plenty of phones that run it and has been for some time, while the N900 is the first Maemo phone, the first Maemo 5 device and was just (barely) released.