I think it's because online, each of us is disembodied and anonymous. We're a nobody. And nobody wants that. So the quickest way to become somebody is to join a group -- ideally a group that seems to be a winner, that offers easy compelling answers to hard problems... especially answers that somehow favor you and others like you. That's populism. Populists are fearless and decisive and sure they know what's true and right. They also know that everybody who's not on their team doesn't get it. So the quicker you join them, the sooner you become a somebody, a player. And if everyone else like you joins that team and you don't, you could degenerate into something worse than a nobody... an enemy.
Combine that with where fame (and leadership?) seems to have evolved in our culture -- where becoming famous is the result of doing whatever it takes to attract attention. Even if if you lack any substance at all, if you somehow stand out, now you're no longer a nobody. You can't be ignored any more. And in the echo chambers of today's hollowed-out media and bored internet, that seems to be enough for most folks-who'd-rather-not-think to see you as outstanding, and an opinion leader.
Those two phenomena don't sound like much, but they do help explain how an empty angry bombastic boob could become President of the United States of America and split the country like nothing has since slavery.