What do you mean “why”?
You understand, i assume, that this arrangement was not the result of moderator fiat, or specific rules. It’s just how those particular discussions happen to have developed, based on the preferences of the people involved, and the more general inertia that often accompanies this sort of thing.
Historically, on these boards, it often seems to come down to how popular the shows are. Shows that have a large number of fans, and that generate a lot of discussion, often end up with a new thread for each episode. Look at Game of Thrones, or The Walking Dead, or Breaking Bad, as examples.
Shows with fewer viewers often just need a thread per season, or even, in some cases, a thread for the whole show. I was one of the few people around here who was interested enough to talk about Southland, and we only ever had a couple of threads in total about the show. Back when it was fresh and new, Project Runway used to get a new thread for each episode; now it gets one per season.
There is, if you’re interested, a relatively simple way to fix the problem.
If that doesn’t appeal to you, then i suggest you try what i do in Project Runway threads if i’ve fallen behind in my viewing: scroll through the thread looking at the post dates, so that you can read what people have to say about the episodes that you have watched, and don’t get spoilered for the episodes that you haven’t.