POTUS has to pick his fights.  Bumpstocks were not worth fighting because they were modifications to weapons. Fully-automatic is not a good use of ammunition as the recoil drift throws off targeting. Plus bump stocks don't allow time for barrel cooling which can cause jams. SCOTUS allowed sawed-off shotgun modifications because the police and military were using them. The police and military arsenal's aren't using bumpstocks. If Trump had lost the gun control battle on Bump Stocks he could loose bigger battles on other things.