The N900 was never intended to be a mainstream device, that's why there was no marketing for it. Sales for the N900 far exceeded Nokia's expectations, so from Nokia's point of view the N900 was a success.

And more on topic, a point no-one else has brought up is, Maemo was a Nokia project, even if it is open source (and some key parts aren't e.g. the Phone app and the Messaging app), that Maemo was developed and controlled by Nokia was a deterrent to other phone manufacturers making use of it. This is why the merger of Maemo and Moblin to create Meego and more importantly handing Meego to the Linux Foundation to develop was necessary, so other manufacturers don't have to worry about being dependent on something made by one of their competitors. With Android, there was never an issue of phone manufacturer's using a product made by one of their competitors.

Also read this for some insight to why Maemo hasn't been given enough attention by Nokia.