Because that's not what it's doing, any reasonably person can tell. I think it was highly inappropriate, juvenile, and possibly indication of his mental state but it's not "promoting violence". We should be outraged but make sure it's at the right thing, that our president is a senile old man acting like a kid and doing everything except for his job.
When you start reaching like that it sucks any validity of the criticism out for most people and they view it as faux-outrage and another instance of crying wolf over nothing.
And I would say you are wrong. It's part of a consistent and significant narrative from Dump against the media. Here it shows an outright simulated act of violence against the media. People assess this from the likely standpoint of someone who would not act out violently based on imagery or propaganda they've ingested. This ignores the context of people who do act out irrationally and become violent based on ingesting propaganda. Just because we cannot understand being driven to act this way from this sort of content, does not mean everyone is the same.
Considering Dump is the POTUS his narrative carries significantly more weight and authority. Some might say well he's a fool and no one takes him seriously. To that we can say over 60,000,000 people voted for him. There is clear evidence many Americans are being radicalized by Trump's and the alt-right's narratives. So imagery like this can and will promote violence among the segment of that group that is irrational and unhinged.
This is similar to seeing a bunch of ISIS propaganda on the internet. Most people exposed will not become violent and act out, the irrational minority may and some of them do. Trump's twitter and the garbage he and his administration push on select media outlets is no different; it's propaganda. Almost everything that comes out of his administration appears to be lies and twisting of facts. They are pushing a propaganda program against their own people. Consider where this propaganda may have
originated from. A shithole on reddit for conspiracy theory driven degenerate pieces of shit who idolize Trump as if he were some sort of deity. Anyone who has seen that subreddit knows it is mostly comprised of human filth, these are the sorts of people Trump is propagandizing to. They are already unstable.
Trump's behaviour is a furtherance of his attack the media narrative. In the same vein as Hitler's lugenpresse initiatives in the 30s. Someone is going to eventually attack someone in the media, wait they already have, an American congressman. Someone else will at some point though and then the bullshit will shift to 'they're mentally ill, Trump's narrative had no effect on this whatsoever'