According to Relic in one of the first post WFA streams the reason why the .50 cal is more fragile is that it's better damage and suppression is balanced by it's fragility.
Honestly I'm just more concerned with how easy it is to kill the crew on the .50 cal/MG42 while standing directly in it's field of fire.
That would be an excuse if it costed 240mp.
Hint: it doesn't.