This is my perception of one possible contributing factor. Note that my perception of what's going on in the community is likely highly colored by my own feelings and behavior patterns.
Coincident with higher activity levels on our core Q&A site, Mi Yodeya used to have significantly higher levels of community activity, including banter in our chat rooms and cooperation on communal projects such as Mi Yodeya Publications and an ambitious siyum project.
Monica Cellio was one of our moderators and also a leader in many of these communal activities. She was also a moderator of a bunch of other Stack Exchange communities. In late 2019, the Stack Exchange corporation summarily and permanently fired Monica from her moderator positions for reasons that many Stack Exchange participants consider unjust and injudicious and in a manner that many consider unfair and damaging. This and other destructive actions that the company took at the time led to an extended period of bad feelings across the Stack Exchange network.
I believe that community activity at Mi Yodeya dropped off significantly around the time of Monica's firing, and I think that there is a causal relationship between these phenomena. I suspect that some of the most active members of the community (me included) developed negative associations with this website and therefore lost some of their previous enthusiasm for building community here. That, combined with the loss of Monica herself as a leader and participant, may have contributed significantly to a drop-off in community and Q&A activity here and elsewhere on the network.