I think it's a liability issue. If everything is less than 5' depth, nobody is tempted to dive (well there will always be some). Too many diving accidents happened at pools that allowed diving in the past, resulting is some deaths and cripplings Also at a 5' depth, most adults no matter whther they can swim or not, can touch bottom and keep heads above water. I'm sure this makes the lifeguards' jobs a little easier.
Then again, I am just theorizing here. Anyone with hard facts, please contradict away (I will not be offended).