I’m sure a vast inventory was a factor, but that begs the question, what would be the “best” sniper round?
I’d guess a lot depends on the venue. Range is a jungle is very different from an open desert. A .22LR is a fine sniper round when the sniper can sneak up and press the barrel to the back of the target’s head, .50 Cal BMG is great at a couple kilometers.
.300BLK is shaping up nicely, comparable to .308 (standard US sniper round) but with excellent subsonic capability (silenced) and usable in the standard AR platform too.
CheyTac (?) .408 is an excellent very long range round, but rather specialized. Better than .50BMG.