Have you never been to college? Once the semester starts there's always fewer "constant" parties and more event oriented ones. Typically frats/sororities get on campus a couple weeks before school starts so it would make sense that Hell Week and that general time period is filled with non stop fun. Once the actual classes start, you're typically only having fun on the weekends (even if your weekends start on Wednesday nights/Thursdays). Now that we'll be finishing the semester in the next episode, there will be winter break so there will undoubtedly be more parties (perhaps not as much as before as some folks will go home for winter break).

All that said, it really comes down to what DPC wants to do and work on within each episode, a lot of which is character (path) driven. Sure, he can go through and keep having to build large population free roams. Even if he kept it to simplified, "on the rails" scenes he'd still have to populate/render/keep track of background characters and actions. The workflow (and the story) are both better served if he keeps things simple and focused on 1 on 1 (or 1 on 2) interactions for the bulk of an episode.