I think that there may just be easier targets. A few years back I spent a night in jail (don't ask), in the same cell there was a guy who was quite happy to talk about his M.O., he specialized in lifting the floor safes from small strip mall fast food joints (places like Subway). Apparently they don't keep much in them overnight, but because they close long after banks do, it's just easier to keep the till float and evening takings in the floor overnight. He gave a startlingly detailed description (he was in withdrawal and just wanted any distraction) of how quickly and easily he could get into the unit, find the safe, pull it out of the floor and be out of there in just a few minutes. He claimed between $1K and $3K was usually in there... at which point why bother with pulling over a bank.
In his own words he got caught because he was "too successful, I was taking so many drugs I got stupid, the cops pulled me over with safes and paraphernalia in the trunk"