Well, here's another comment that I posted about that in this thread, I think it explains it pretty well.

I can agree that it only seems like they care about scanlating bigger and more popular series, I'm sure quite a lot of people who don't do scanlation, and probably even quite a lot of scanlators would agree.

Thought the thing you might want to consider is that none of them get paid, they spend hours working on these chapters themselves, and I'm sure quite a lot of them love the series.

Why dont they work on other smaller series they also love, you might ask. Well the thing thing that I'm sure a large majority of speed scanlators like (I can tell you this from my past experience in speed scanlation), is people reading their releases, a small view number on mangadex, an upvote on reddit, stuff like that is why they are in these big groups that are willing to work on these things.

I haven't been scanlating in a few years now, but I can assure you that most scanlators, even if they work on big series, earn no money, if anything, they lose money and time.