It's a great question, and there are probably a lot of complicating factors, including but not limitted to the whole Monica kerfuffle, COVID, and various site scope discussions where parties disagreeing with the main body of users saw fit to take their energies elsewhere.
But probably the big answer lies in the private beta excitement. It's pretty typical on new sites to get a huge spike of activity, as people jump on the bandwagon of an exciting new site. But as has been the case with many beta sites across the network, those aren't necessarily the people who are really passionate about the subject to the point that they will regularly take time out of their schedules to ask / answer questions or even vote.
Activity on beta sites tends to see huge spikes at milestone time periods, such as when the site first goes into private beta, when you get out of private beta, etc. But once that hype is over, you get into the slow growth period where you build a base of people who really know the subject well and are willing to participate.
That said, if users come here, post one question/answer that gets one vote sometime a couple days later, it's not really condusive to growing the user base. Voting is really important. Unfortunately, we don't have a lot of dedicated voters who will recognize and vote up great material.
Basically, probably a lot of different things, but it's not unusual for a beta site to see reduced activity in the years following its launch until it can attract active experts. It could also be noted that back in those days there was a concerted effort to advertise the site elsewhere, whereas there has been little discussion about it recently. I guess reviving the chat wouldn't be a bad thing really.
Anyway kinda rambling, but those are my basic thoughts on the matter.