N5296S, I was going to answer along the lines that you need to avoid 'curl over' as the air descends on the downwind side of the mountain. Very dangerous in strong wind conditions.
Tell me about it. I once hit a full-blown rotor flying over the 8000' ridge north of LA. I lost 2000' in about 10 seconds.
I wasn't asking "why shouldn't you fly close to the terrain in mountains", which is somewhat obvious, at least unless you are in close to dead calm conditions. I was asking why the MSA is 1000' higher in mountains. I guess I somewhat answered my own question there though.