People have listed some good reasons, but no one hit the real painful and unpleasant reason:

People are negative about it because they missed the ride up. It was at $1k earlier this year when these people started dissing it and calling it a Ponzi scheme, and now it's at $15k. Those people missed out a 15x gain, something most startups they've labored their life over can't do. It's human nature to be jealous