Why would they do it now and not before?
Right now they have the Presidency, Congress, and SCOTUS. They have a base with enough people who are willing to do anything for Trump. They have an overall constituency willing to suspend elections.
This thing about violence coming from the left may be true under the circumstance of a sudden loss of our Constitutional rights. But that's not mindless lashing out over the loss of an election. Among other things our Constitution is based on is Locke's Second Treatise in which Locke states that when a government ceases to represent the people/fulfill what's it's supposed to under its founding document, the people have a right to overthrow it. I believe in that.
Have we reached that point yet? I don't think so. But I do think we're closer than we've ever been since the McCarthy hearings of the 1950s. Fortunately for the nation, at that time, good people in positions of power finally stood up to McCarthy and left him as a filthy example of what can happen when our rights of speech and association cease to exist. But it also shows that it can be stopped. However, the situation is different now in that people believe in provably untrue things; and it doesn't matter that Trump lies to his people. They don't think he's lying to them. There's no reason to post the litany of blatant fabrications because they're firmly on the record. But this phenomenon we're seeing right now is something I'm unaware of in this nation's history--at least not on this scale.
If for example, elections are suspended in November, then violence is a wholly appropriate and lawful exercise of the people's will. There is simply no other answer.
The futility of it is another matter, but I accept that.