Is it bacuse they are smaller? It seems until recently, plasma was it in the above 42" market for thinness.
Not sure if this is true but I was told that plasmas weigh more than LCDs, although if true, should not prevent wall mounts. The industry encourages wall mounting for the "cool" factor.
I saw a 42 inch plasma that weighed 82 pounds and a 42 inch LCD that weighed 60 pounds. I am not sure if this is a consistent thing or not. Either way, it does not matter as the hardware and mounting procedures still allow either one to be wall mounted.