Because the many myths & illusions of America are losing their hold over people but there is no large, ever-present socialist movement to educate them. To people like you and me Hasan isn't that radical but for people who are fresh off the teat of the propaganda machine, whose eyes have yet to adjust to the bright Sunny sky, he and people like him are a breath of fresh air in a city full of smog. The mistake I think some people make is that just because somebody started with Hasan doesn't mean they won't outgrow him. Succdems are still succdems at the end of the day, but they are good for waking people up. I myself used to be a Bernie girl and now Bernie is too right-wing for me. It's always refreshing to discover people with a platform who share your beliefs to help convince you that you're not crazy and to give you more confidence in what you believe in. For me that was Bernie and for some that is Mamdani or Hasan.
Honestly, Hasan has probably led more people to radicalizing than every US socialist party combined. Which is tragic in its own way but without people like him forcing liberals to actually use their critical thinking skills they're never going to find their way to Marxism. Hasan especially is important here since unlike a lot of SocDems he doesn't write off "Tankies" automatically.
I don't hate the guy and I hope he gets more radical with time. I appreciate what he's doing, nevertheless.