In some jurisdictions, the local gun law requirements for a pawnshop are more restrictive than the requirements for a regular gun shop or sporting goods store. I suspect, being Las Vegas, a pawnshop that handles modern firearms is viewed as shady. The 1968 Gun Control Act that established 1898 as the cut-off for antique guns, was predicated on prejudice against pawnshops, among with other assumptions about guns and crime.
Around here, pawnshops that deal in guns have to deal in a lot of guns to make a profit. So if a pawnshop wants to deal in general collectibles, antiques and art, and don't want to become a gun shop that handles pawns on the side, they might simply not want to get into handling modern firearms.