The iPad runs on the iPhone OS so essentially its the same one thats been around for the last few years. After some research it seems as though the 3GS had N64 emulation as of late last year so its no surprise the iPad got it so quickly.

The N900 however is a fairly new device with alot of kinks so its still a bit of a challenge to do such things.

Lastly, The iPhone is owned by more than 50x the N900 owners so its no surprise that their development is faster.