- because a part of those going to buy a N900 don't own a previous maemo device, and don't want to spend 200-300€ more on top of 600€
- because there's a difference between carrying a N900 and carry a N900 plus something even bigger and heavier
- especially because you often don't know when you'll need that feature and don't want to carry the N8*0 around always just in case...
- because for that price people have the right to expect that feature included (maybe not officially, but with a hardware that makes it available in some way, as a software hack or/and modified cable)
- because there wouldn't be a thread "N900 will not allow USB OTG!" with 30 pages of people searching for a solution if everybody could so simply get a N8*0 to carry around and solve the problem

didn't think it was hard to guess any of these reasons that just came up in my mind, looked like common sense; sorry
and sorry for not understanding the joke