For Facebook the move is an attempt to maintain the integrity of its advertising platform which in the past 12 months has been criticized for allowing Russian propaganda and other content deemed inappropriate.
They just don't want any issues again that may bring them down or the authorities would call out Zuckerberg again for investigation. Just like what twitter and google ads is doing because they don't want to cause any issues since cryptocurrencies is still a hot topic.
Recently though, I have seen some crypto related ads so I don't know if they lift the ban or they just weren't able to monitor the advertisement that it is related to cryptocurrency.
Such a policy of Facebook. And probably in fact, one person - Mark Zuckerberg ... This is his subjective opinion, which he decided to implement in this way. It seems to me not advisable to prohibit advertising crypto-currencies - for this future, and when there is a ban - there will still be other options for advertising crypto, with Facebook or without Facebook.
There are other options but due to this move, bounty hunters are more needed because the promotion of bounty hunters would only be the way to promote on those sites like facebook and twitter. Youtube didn't implement the restriction so many are still promoting their ICOs there as well as in the crypto related websites.