Because I liked the idea of an open source OS and go away from Symbian as it was getting a bit long on the tooth.
There was no worthy successor to my Nokia N95 for my taste and the Nokia N900 sort of had my trust in replacing it.
I noticed more and more reasons why I got the N900 once I actually owned it and started using it.
Never thought I would adjust to a QWERTY keyboard so quickly and never wanting to go back to T9 on a keypad ever again, to name one thing that I saw as an obstacle and challenge at first.
Plus it meets the criteria I had for it: do everything that my Nokia N95 does and preferably even better!
And everything I used on the N95 it indeed does better and some things WAY better.
Glad I went for the Nokia N900 and not missing my N95 at all!