Firstly youd be surprised how many millienials arent bernie fans. I currently live in Kentucky and Sanders support is maybe 20% of millienials, going by a cursory glance. Otherwise even a trash state like Kentucky, after generations of being a QoL shythole (despite having the most powerful senator in US history) still votes substantially red. Even in the college towns!
Very few people, even the college educated, can articulate what a Democratic Socialists platform is, and why it can fix a lot of the persistent problems in this country.
Understand, FOR 60 YEARS, there has been a organized and deliberate propaganda scheme against any political stances that call for higher taxation rates of the rich, organizing labor/workers, nationalizing industries, reinvesting tax dollars into "people building" (education, healthcare, ecology, housing, infrastructure, etc) instead of the DoD, corporate welfare, pork and barrel projects, etc. And thats not even factoring how the USA was founded on the principle of FYGM and "Watch out for those nigg344$/Jews/Fakkits/Messicans/Polacks/Wops/Ayrabs"
Bottomline, even with all of the strides we have made, this countrys populace has a long way to go before buying into a true progressive stance. We first have to believe in solidarity with one another and believe in investing in our people, even if we arent alive to see the benefits of it.
But turnout for young conservative voters is low as well. so low voter turnout is still a young people issue accross all political spectrum.