Because they are pretty much the only place to host onion sites, at least for a long time they were the only place offering to host sites for free. I would avoid using any hosting provider like freedom hosting and opt to host things yourself, but they have been around for a long time and they host a LOT of EXTREMELY illegal things and would likely go to prison for life if ever located, they must be taking good security precautions, but they say straight up that if they are ever compromised they will cooperate fully with law enforcement to get the best outcome for themselves as possible.