1
1) Because they dislike HRC for her apparent dishonesty and her long-running history of being financed by Wall Street.
2) Because they are true believers in socialism and perceive Sanders to be the one carrying water honestly for the cause.
or
3) Because young people like underdogs. This is a bigger thing than you may realize - I remember, in my voting-aged youth, I was always attracted to the conservative candidate that was considered an outsider/underdog, because I wanted to be part of something big. Sanders is calling his campaign a "political revolution"...young people want to be part of a revolution because it makes their lives feel purposeful.