I think it's because anybody who has tried it finds it natural and easy. Sure you could trace a spiral in/out like on the N900 or hit +/- buttons with a pen but it's by far one of the most natural and obvious gestures.

Also, haven't we solved the capacitive v resistive thing already. Capacitive won. No more having to hit weeny buttons with your fingernail because you've lost your pen.

For those of you who want to use pens but still have decent multitouch gestures, there's hybrid screens that layer a Wacom panel under the capacitive layer like the Galaxy Note. These are infinitely better than resistive in day to day use.

I don't see why the N900 screen gets glowing reviews either. Of the two I had, they regularly went out of calibration, scratched easily and had awful viewing angles. Even my SE P910i had a better screen.