They are either directly affected by massive school loan debt or have close friends and family who are. That makes the appeal of free education and the tie in with the Socialist message very strong. JMO
These millennials are college age. Most of those kids think socialism is great. Free everything.
Of course nothing is free and as soon as they are out of college, get a job, have a mortgage, car payment and kids to pay for they find out about the real world.
The world where you have to work and earn a living. Free stuff ain't free and it doesn't take them long to find that out.