One, important factor, or let's say THE most important factor for n900 to choose a resistive screen, is the accuracy.

n900 uses predominantly the same software that runs on your PC, which are written for mouse usage, and therefore, require pixel-perfect accuracy.
If n900 had a capacitive screen, all software would need rewritten UIs, and most games wouldn't work unless they were rewritten entirely.
The whole point of the n900 was to use existing software, not create another symbian.

I still believe that the future of mobile phones lies in running the exact same software as on PCs, not variants of it.
So, currently, that implies a resistive screen.
Unless everyone suddenly decides to rewrite their X11 software to support mouse *and* a tiny resolution mode (possibly with multitouch)

Not to mention multitouch is a very difficult nut to crack, since most mouse algorithms rely on the fact that the cursor can only be in one place at any time. Making software multitouch aware will surely degrade performance for people only using a single cursor.
It's also likely that people will start pushing different UI widgets into separate threads.