Because I just couldn't put up with my 5800XM any more. It was a very frustrating phone to use. The notifications that block your usage of the phone until they disappear was a very big nuisance. So far I'm very very happy to be using the N900, even with the bugs and lack of turn by turn navigation it's still such a huge improvement over the 5800 that I'm a happy camper.

I could not have waited for a future gen device. I was considering a Motorola Milestone, but stuck with Nokia.