How is it that non reliable mangakas get work again and again?
Business-wise, that is not very important because as long as these mangaka still sells, publishing companies would still like to sign a deal with them. To stop these kinds of mangaka from keep having jobs, then the market must reject their new project. Only then would a publishing company be unwilling to sign them.

Now, most readers are just casual readers. They just read when they like it and are not that attached to a series very much. Only hardcore fans would follow and know everything about a series. Thus for most reader, whether this mangaka is known for being such a person is not very important.

In fact many don't bother knowing who the mangaka is. My sister, who is not an otaku although she ocassionally read manga, knows Naruto, Fairy Tail, Bleach, One Piece, and many other mangas. But if you tell her Masashi Kishimoto, Hiro Mashima, Kubo Tite, Oda, then she'll say, "who's that?" The only mangaka she knows is Fujiko F. Fujio (because it's Doraemon). What they need to know is just the title of the manga and whether it is interesting or not.