Because it's one more chink in the armor of 'suppressors are evil, no-good, really bad things.'

It adds one more bit of weight to point at and say, "look, we don't even put any unusual export controls on these. We treat them the same as any firearm export just like most every European countries do. So why don't we go all the way and treat them just the same in every respect?"

Which leads to deregulation.