I actually don't think the "Social Justice Warriors" are the problem in this case. (you can blame them for many things, but not this, in my opinion)

It simply was that the "Democratic" party wanted Clinton, so they use "Super Delegates" to make her the candidate even though Sanders would have had _much_ better chances.
The "Democrats" knew that Clinton would definitely loose against any "sane" Republican, as she is extremely unpopular and represents virtually everything that's wrong with US politics.
In such a situation you can control your enemy to some extend by focussing your attention. You can ignore some candidates while reacting to others, thus making them appear more important, that's how we got Trump.

Now there is a theory that Trump didn't actually want to be president, but just do some brand management on his brand and then loose. (this is a non vital fact in my argument)

There are certainly people who see no hope in this election, so everything they see as an option is to destroy the system. They do this by choosing the worst possible candidate for the job. Now everything Trump does "wrong" will convince those people that Trump is the right person.

You'll notice that the political spectrum doesn't have anything to do with that. There was no "left" candidate except for Sanders. Without Trump, Clinton would have been indistinguishable from the republican candidates.