I think one of the reasons is that these ICOs are really enticing and promises big returns. Most of of them are lending platforms that encourages beginners to invest.
And maybe this is why bitcoin banned it, because of the negative effects they can get if they accepted scams advertisement that may lead a blame to them. But bitcoin really has a huge potential. Facebook did not realize it, they should need to be filtered in able to avoid unreliable ads. Facebook can earn more with bitcoin they need to think about it once again.