Because Maemo 4.x is not user-friendly enough and is not a 'smartphone' OS whereas Maemo 5.0 has not shipped on Nokia N900 yet. Once a product is out in the wild the traction will start.

Rome wasn't build in one day. When Android was still being designed it didn't gather that much traction either. If you look at it from this point of view, and see the media coverage N900/Maemo 5 already received, I'd say things are rather looking good.

Especially when Nokia fixes issues reported by community. I assume here the fixes concern important issues. The initial out of the box experience is very important. Especially with a new product which is 'very different'. For example, iPhone 3G coverage was initially bad, and N97 camera problems also existed