Why not top quality side pull brakes for MTN.
You always see side pull brakes with road bikes they sure look a lot mor expensive looking then those cheap side pull brakes at dept. store on the regular steel mountain bikes...Why can they make a expensive side pull brake for MTN. bicycles
There easy to use?
The reason MTB don't use them is primarily for mud clearance and tire clearance. You would have to make them extremely large to fit larger tires and to allow mud clearance on top fo that. Even Cyclocross bikes dont' use sidepull brakes for those reasons. Technicaly would be possible, but to make them that large ther would be a a big weight penalty and I would think woudl get clogged up with dirt/mud alot easier.
Disc brakes...the market is shrinking on expensive rim brakes so I imagine the investment just isn't there...
You can make sidepull brakes with more clearance, but the longer the arms, the more flex they have. Shimano make a long drop version for 32mm tyres + fenders, but that is the longest you can sensibly go. Its a really nice brake, and I use it for road and light trail riding.
You have a choice of cantelevers, V brakes, roller cams, hydraulics and various disk brakes and drum brakes. There really is no call for a long sidepulls.