Actually, soldiers had been using scoped M2 HMGs as sniper rifles since Korea.[1] After Hathcock's success with one, Barrett decided to build a real sniper rifle around the cartridge.
There's a big difference between a sniper being issued a 30 pound rifle he can carry, and the mode in which Hathcock used M2s, by borrowing the use of one if it was handy (128 pounds with tripod if not mounted on a vehicle).
I agree, which is why Barrett made those rifles. My point was that the .50 BMG had been recognized by the Army as an effective long range marksman's round, to the point where they were procuring and issuing scopes for the M2, 30 years before Barrett made their first rifle.
But the military did nothing beyond procuring scopes for M2s, effectively keeping the powerful .50BMG out of snipers' hands as the M2 was a heavy crew-seved weapon (vehicle or tripod mounted), unsuitable for a lone sniper to haul for miles. Sure, Hathcock et al could make amazing shots with it, but only from where it could be little farther than driven into position. It took the civilian market to translate it to the true sniper's rifle prevalent today.
[1] https://en.wikipedia.org/wiki/M2_Browning#M2_as_a_sniper_rif...