Basically it's this. Nokia have been working on maemo for a number of years to the point where it is stable and mature. The smartphone market is very competitive and competition is fierce.

Nokia thought that cracking this market and exploiting it was not hard enough as it is and decided to abandon maemo and joined a new system instead - called MeeGo.

Meego does the same thing as maemo but is more open source and development is open to everyone (one day).

Meego is the best because maemo does not sound cool enough and maemo is really old now.

There is a large collection of software for maemo written by many developers who spend significant amounts of time writing new software with loads of features that make the n900 so interesting to own.

With meego there is a probability that much of the current software will have to be changed to run properly on Meego. Developers hate writing new features and prefer spending their time on re-testing and tweaking their software to run on Meego. Why fix bugs when you can spend time constantly changing your software on an ever-changing platform?

Maemo is based on gtk and Meego is based on QT. Gtk & QT make the software on your phone look pretty and help developers write cool software.

Nokia chose QT because it's the best.