The biggest reason not to use the 50 cal. M-82 to engage enemy personel was that the early m-82's that were issued to the Gulf, was "Accuracy"!!!
These early rifles shot between 2 to 3 MOA depending on the ammo (No match ammo available! When it was it was very scarce! Mostly issued to the Seal's, Marine's never got it! delinked only! This was the same for the M-40's, delinked ammo!). So to engage targets beyond 1000 yards they had to Minute of Tank size.
To compare Rob Furlongs shot's in the Afgan. to Carlos Hathcock shot's in Vietnam!
Well let's look at them!
Carlos, shooting a M-2 from a "fixed" tripod, an area weapon, aimed at a specific point within the area, pre-shot to confirm zero, all he had to do was wait until the target reached a certin point within the cross hair, compensate for the wind and engage.
Rob on the other hand used a very job specific hand built Precision Rifle with Match grade ammo, with no chance to "practice shoot" the target. He had to engage his targets with the aid of his spotter's.
Both shooters produced amazing accomplishments with his given rifle, but given an early M-82 at the distance's they engaged their target's, chances are they probably would not have set these records.
Bottomline to your question, "Accuracy" was the reason!! Nothing more than that!
Hope this helps! Not trying to start any slinging!
Cheers