Because at a very superficial level it seems a solution to the inequalities inherent to free markets - you can't have winners without losers. Younger people are particularly prone to seeing communism as an amazing solution that simply has not been done right, seeing themselves as having found the one true implementation that would actually work. The reason it skews more this way for younger people is a combination of lack of experience, limited exposure to working life, incomplete brain development, and academia presenting both ideology and an artificial world on which they base their views.

After World War 2 everybody quite rightly condemned National Socialism, with a great deal of research into how ostensibly normal people could be part of it. Given the political leanings of the most prominent researchers, it's not a surprise that they considered this exclusively a right-wing problem. It was only later that further research into the authoritarian personality considered the left. With decades of apologising for and concealing the obvious parallels in communism and other far left ideologies, the left never saw the scrutiny it should have received while filling mass graves pretty much everywhere it was tried. We're now left with people not realising how terrible communism is. Had things been done right, extolling communism would be viewed the way we view people who quibble over the numbers exterminated by the National Socialists.