Are you hoping for simple answers to these questions? Motivations for randonneuring are myriad and personal. I can't really explain why I like doing it...
If my region is any indication, a lot of RBAs haven't even posted their 2022 schedule yet, so I would take your search results with a grain of salt.
If 300s are indeed less popular, perhaps they're a "mushy-middle" between 200s and 400s. You can bust out a 200k and still have time to do other things with your day. A 400k is something you go ahead and dedicate a whole day to, and the fatigue starts to get "interesting." 300s might well be some people's favorite, but they just strike me as a part of the series to get done.
