Because of all the civilian snipers, the military adopted it?
Yes. Substitute "target shooter" for "sniper" and that was basically what happened. These guns were developed mostly by and for civilians.
It might have been designed for machinegunning nearly a century ago, but it has held the distance sniping record for the last 40 years.
That's exactly my point. It's been accepted simply because there wasn't anything else in that niche. If you like .30 hunting cartridges, you have hundreds to choose from. If you like big bore, 1000m+ cartridges up until recently you only had one. Imagine what they could have done with a better cartridge. Today we have cartridges with higher velocities and higher BC's and that translates into less wind drift, less drop and greater range before the bullet goes transonic.