Also, mountain wheels are usually disc only, and thus the rims weigh less (no braking surface). And, non-track wheelset weights are usually reported without QR's, while track wheelset weights usually include the track nuts also. Solid axles add a lot of weight too.

All told, when you look at the quantity of MTB wheelsets sold vs. the number of track wheelsets sold, it's no wonder MTB wheels get more R&D.