I think most of those who can provide quality comments leave because now there are many scam and deceptive projects, maybe they don't like this, feel what they have done is in vain so they decide to leave this world of bounties. Moreover, in 2018 the existing projects were good and profitable, much different from now because of the many scams, although many projects were promoted but it did not rule out that one of the promoted projects was a scam.
Most of quality posters are joining the bitcoin-paying campaigns. Though I am not saying that a lot are trashy in bounty campaigns but they have less restrictions when it comes to accepting participants. So for others who feel they are not getting fair compensation, they are sticking to btc campaign even if the pay is not that much. At least, they know they will get their payment as agreed.