The reasons snipers used the .308 were two-fold, the .308 was develped for the military to replace the 30-06, they found that most soldiers were shooting at 2-300 yards, not 5-600, so faster velocities weren't really needed...Also with the .308 the ammo is lighter, so they could carry more rounds, this is why it was develped...In addition, the .308 being a shorter cartridge is inherently more accurate than either the .270 or the 30-06...Same reason the WSM cartridges were develped...

In a sniper situation, bullet drop is not as important because they use a tactical scope, they simply dial the scope into the range and hold dead on.