I'm not entirely sure why that seems to be the case, outside of the obvious issues in posting general threads on /co/ for whatever reason the mods want to dream up.
Outside of that, some reasons for the slow threads could be linked with the weekly schedule not really providing enough content to talk about outside of the first couple threads. This is particularly apparent in the wake of the hiatus, where every dredge of discussion had to be used up to fuel the threads just on the weekends. Much of the discussion in those hiatus threads also ended in "we'll have to wait for confirmation or more info in season 2," so those strings of discussion are still open until we get more solid info.
Another reason could be the slow start to the season, whilst this is debatable I personally feel a lot of the episodes so far have been a bit unfocused and at times painfully average. Some episodes have been fantastic however, including the two most recent episodes so perhaps participation will pick up.
Another possibility is the lack of development for the female secondary characters, given qts make up a lot of the impetus for people watching cartoons or anime nowadays. Upcoming episodes look to be touching on said secondary characters so, again, hopefully that will help participation in threads, given the importance of developing secondary characters to supplement both the plots and main characters, both of whom have had considerable development in the first season.
Lastly, most of season 2's episodes have been primarily on Earth. Whilst this is useful for world building, it does limit a lot of what makes Star vs. so interesting to discuss in the form of the magical dimensions they visit. Should this get some more focus the amount of possible discussion topics should increase.
tl;dr: Hiatus threads burned up content and mods hate generals. Season 2 had a somewhat inconsistent/weak start with a lack of focus at times, teamed with not enough qts or magical adventures.