It's the marketing!

iPhone and Androids are pushed out in the consumer market with strong marketing hype. Not many people know about the N900 because there's little/no marketing activity (compared with what Apple did for iPhone). Or at least in Nokia's perspective (justification), the N900 was meant for niche market.

Remember what Sony Ericsson did for one of it's model with James Bond (I can't recall which was it)? The only place people will hear about the N900 is from this forum.

Marketing strategy! That's how you effectively sell your products