Because the Left has never totally rejected Communism

That's why.  So a report found that one third of millennials believe George W. Bush killed more people than Stalin.  As I said here, the Left has always cast a longing eye leftward toward that old Communist religion.  There's something about Communism that tugs at the heart strings.  Generally, even if they don't long for a good old Bolshevik revolution, they nonetheless attribute the worst of Communism to US meddling.  We are the cause of all problems after all.

That millennials would be unaware of the true scope of Communism is hardly surprising.  I've talked to under-30s before who are fully convinced that the Cold War involved brave Soviets fighting back the evil US Industrial War Machine. 

Likewise, what is the casualty count up to now?  Did our Iraq invasions kill hundreds of millions?  Or is it still only in the tens of millions?  I can never keep track.  I know we're up to around a billion American Indians killed by genocidal Europeans coming over to destroy the American environment.  That doesn't count the hundreds of millions of Africans killed by slave owners who, apparently, thought nothing of wiping out their investments for the sheer delight of spending money just to wipe out their investments.

It's funny.  Going through college, one of the things skeptics always dismissed about the historicity of the biblical narrative was the figures and numbers.  You know, millions here, tens of thousands there.  Clear and obvious exaggeration on the authors' parts.  The ancient near east had nowhere near enough people to account for such stats. 

And yet when I see the way inflated figures are thrown around at US history, mixed with the fact that the left downplays, if not outright ignores, the Communist track record, it makes total sense that millennials would be so unaware.  We're we reached the nadir of historical studies.  Don't blame the students of such a time for being so ignorant.

Oh, and saying Communism is dangerous because Donald Trump and others who are not liberal are the real Bolsheviks, only muddies the waters of understanding.  So again, cut the young'uns some slack.