Because in my opinion maemo is the better OS and N900 has more option like sdcard, keyboard, infrared. Only the processor is a little bit weak. I prefer a computer with more options than a faster one with less options and less user rights. So why buy a computer when it is faster and has more memory when you can do nothing. N900=computer with phone function N9=a cell phone.
But you can hope that hackers will free the N9 when it is not too late then more users will buy and more apps will be created and ported