maybe because they are so big and most of the advertisers advertise using ad management tool, so a tool and who knows no human interactions. Given that, they didn't know if a advertiser is legitimate or not. I think is the same with fake news which in most of cases are promoted , so someone pay for them, and Facebook is not able to stop them.
Given that they are not able to be selective they ban every add that has the word: crypto, ICO, blockchain and so on.

Maybe I'm wrong but for me the biggest Facebook problem are the fake news and not the concurrency.