one factor
Ever heard of neck dive? This is something that happens when your guitar isn't balanced... When you remove for fretting hand from the neck of the guitar, the wieght of the headstock causes it to drop, sometimes a bit, and sometimes nearly all the way to the floor.
A good example that I have seen is the Gibson SG. It has a thinnner body, but still has a large headstock. When you give your fretting arm a rest the heastock sinks. (The special models aren't as prone to this due to thier smaller headstock)
Some people don't mind neck dive, others hate it... To me it would be quite annoying to worry about holding up your guitar AND playing at the same time.