The simplest answer is that they're expensive.
Were NFA '34 and GCA '68 were wiped tomorrow, and erm... "cultural acceptance" of DD's was comparable to that of Title I firearms, I sincerely doubt that the statistical distribution of weapons by type or caliber would change all that much.
Perhaps people would have more stuff like grenades, and more people would play with rounds over .50 BMG, but overall, even without NFA taxes or paperwork, very few of us have the time, the land/space required, or the financial resources to mess with RPG's or a Carl Gustav.
It would be nice to have the option to do so, go back to when "men of means" would equip or better supply their volunteer units, even up to the time of the Civil War, but in terms of actual distribution of such assets, I don't think it would change all that much. While I admit there's few commercial concerns serving the market, the DD space isn't artificially limited like the MG market has been since '86, and from those few folks I do know who play in it, the NFA taxes and paperwork is really the smallest hassle when your weapon costs you a couple hundred bucks a shot.