That and trying to keep people around. If you do shorter seasons it can be easier for someone to jump in to it, rather than this whole back catalogue. Heck, with the new Ahsoka Star Wars series coming up I realize I didn't really watch a lot of the other shows (like Clone Wars, Rebels) and people are like "What do I need to watch to get ready?"True. Longer running franchises like ST or DOCTOR WHO run the risk of appearing daunting to watch. Even more recent ones like STARGATE. ('Recent' as in about 30 years old or less.) SUPERNATURAL can appear very daunting... with 327 episodes, that single show had almost as many episodes as the entire STARGATE franchise.
It doesn't help that people tend to not focus on anything longer than 20 or 30 episodes. I suppose that's also part of the reason for shorter seasons... to compensate for the shorter attention span.
And I'm like, well, nuts. That sounds like homework, My time is limited. How do I put this little bit of a show here, and there, and watch it and try to keep up with something new.
I know it's vogue to blame short attention spans, and the upswing in "short" content, but honestly sometimes the dearth of material is just too much.