Because Trump works hard to convince his followers that Democrats deserve it..

If you thought that Democrats want to force the entire country to starve and then allow MS-13 gang members to come to your community so that they can kill you and your family — which, because we have to keep repeating this, is literally what Trump says — then why wouldn’t you feel that any possible means, including violence, should be employed to stop them? It’s only self-defense.

The politically savvy might dismiss that rhetoric as just crazy hyperbole, but there are people who take it seriously. And even if only a tiny number of Trump supporters go so far as to beat up liberals they encounter or even undertake a campaign of terrorist bombings (granting that we don’t know who sent these bombs, but it would be rather extraordinary if a collection of Trump’s enemies was targeted just by coincidence), that’s only because most people have emotional controls that prevent them from breaking the law in that way and physically harming other people. But not all.

Yes, Democrats have said some intemperate things, too. But Trump has sent the small number of violent people a message. It’s one they might be hearing loud and clear.==============================

Last I checked, it was a dem lunatic who shot a  republican senator.  Also, It was a dem lunatic that attacked Rand Paul.  Wasn't ricin just sent to a Trump cabinet member?  My guess would be a dem lunatic did that.  Did not a sitting dem congresswoman just endorse publicly harassing and accosting republican officials?  Did not dem lunatic mobs do just that without a single peep from anyone in the media or dem poster here that that was a bad idea?   

IF it is a pub/Trump lunatic who is sending these bombs arrest him/them and lock them up for life. End of story.  I don't remember there be any outrage here about this being the end of the republic when any of the previous lunatic dems acted out.