Because Shadow Maul would be seriously overpowered if you had a 100% crit chance. Other PBAoE/cone powers have the same crit chance as SM I beleive.
Yep. SM is still an amazing power - just keep moving and lining up your enemies to get them into the cone, if you hit placate, there's a good chance that the 50% crit will land on one, 1 in 2 in fact 
Why the 50% critical strike chance?
Do the other narrow cone attacks have 50% crit chance? like golden dragonfly and the MA one whose name escapes me.