Because as stupid as a bumpstock is, it is just an attachment. When Trump did that he effectively allowed any future president to ban any attachment at will by fiat. First bumpstocks, then scopes, then foregrips, then picatinny rails, then trigger types, etc.
Because of his sheer incompetence on this one issue, he opened the door for future Presidents to unilaterally ban firearm attachments. We may not care about bumpstocks, but what about 20 round mags? 30 round mags? 100 round drums? Triggers with lighter pull weights? Bipod? Red dots? Etc.
I dont think Trump is going to do any "bad" gun executive action while in office, but the problem is he set the precedent for any future Dem to do it. So, say we get Liz Warren in the White House. What damage can she now do with unilateral firearm attachment bans?
As far as Red flag laws:
"We must make sure that those judged to pose a grave risk to public safety do not have access to firearms and that if they do those firearms can be taken through rapid due process...that is why I have called for red flag laws, also known as extreme risk protection orders" - President Trump