Range more than anything. We have continued to use 7.62 bolt-action sniper rifles even after switching to the C7/5.56 as our primary rifle 35-plus years ago.
We also have some .50 BMG sniper rifles. And I believe they tried the .338 Lapua as well.
As to what our allies do, the U.S., at least, has done exactly the same. The Germans use the .300 Win Mag and the Brits use the .338 Lapua.
We have .300WinMag, 338Lapua, .50BMG, as well as .308Win in the arsenal.
