You can ask why the GOP favors that, but you could also ask why the DNC favors "common sense" regulation usually in the same form, with just a few bump stock or magazine capacity laws attached. The answer is that gun rights are one of the most contentious, single-item policies that voters care about. The NRA is an extremely powerful lobby, not because of contributions from gun manufacturers (which help of course) but because of the millions of die-hard gun owners that vote Republican along that one issue. I personally vote straight-ticket Republican on local representatives because gun rights are the one policy that I know they'll defend, because Republicans that are weak on the issue get kicked out on their ass in most states. Republicans tried bargaining in the past for decades and it meant a consistently eroding access to guns, but the line has been drawn hard now and it hopefully isn't going to budge until the DNC takes the Executive and Judiciary back, if not the Legislative as well.