Why are 3 inches mountain bike handlebar hard to find? I'm looking for a new handlebar, ideally 3 inches. But looking in some of the online shops it seems quite hard to get 31.8mm  bar clamp diameter handlebar with 3" rise. 

Do you know if there is any reason for that?, is it maybe because people change the stem as well?
I can theorize. A higher rise would mean that as you bump along, you apply more torque to the handlebar.

I bet that above a certain threshold, the rise becomes so great that preventing the handlebar from spinning in the stem becomes a real problem.