All you need to do it look at the variety of rifles, ammunition and components available for each and you see the reason why. When you want to shoot cheap, you shoot 308. Here we have large quantities of surplus 308 brass and thirty caliber bullets can be picked up at Wal Mart. Sierra 30 cal 165 grain Game Kings are $30/100. Sierra 264 cal 140 grain Game Kings atr $39/100. Three-oh-eight rifles are found everywhere here, not so 6.5x55. You need to special order one of these. US manufacturers seldom make a run of 6.5x55. Not sure the ever-popular Remington ever made one.
If the 308 is more popular than the 6.5x55 it's not because of any outstanding ballistic advantage. JMHO.~Muir