I don't agree with the conservative calling all the time, but it's easily explainable. Herd mentality. It's safer to live by conventions. It's safer to go with what is conventional. That way, if your decision turns out to be wrong, everyone else was wrong too, thus you don't get blamed.
The fact is, coaches want to win and they want to keep their job. If a coach consistently goes against the norm, those times his decisions fail become reasons for his exit. They're the easily pointed out "mistakes".