A. Japan likes the anime so the anime stays canon with the manga then stops for a season 2 in a couple of years.
B. Japan doesn't like the anime, it's not making money, so they canon until a certain point to where the manga is currently at, then do an anime original ending.
I have to say I really like how fleshed out these side characters are. Originally I thought I would like Mikasa the most but instead I ended up liking some of Eren's trainee squadmates a lot more when they reveal some backstory such as