Fewer people are casting their votes. Check these queries:
As is evident, only 21 million votes were casted in 2019 as against 23 million in 2017. Moreover, the 21 M votes includes posts before 2019 as well. Which means the votes/available posts ratio is declining at an even faster pace.
votes/post | year ------------------ 23 M /33 M | 2016 21 M /47 M | 2019
Considering that cumulative posts by 2020 are well over 50 million, the votes should have also followed the similar upsurge.
Why are people casting fewer votes?
PS: My question isn't specifically about 2020. I understand there are three more months to go in 2020. But even if data till 2019 is taken into account, which has a cumulative post count of 47 M, the point still stands.