Good question, the 7.62x54R is the longest continuously used military round in the world. There are other similar calibers which have better performance. They have developed good rounds from the 54R and likely have a large supply, this could have driven their decision.
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.