Discussion's that I like to know why some anime's don't cover the manga completely

[Bupkis]

So people need to support the anime in order for the anime to continue its like business. So is that like in the same case as in bleach and gantz and other series. I know the bleach manga is finish I wonder why the anime didn't cover everything of the bleach manga same with gantz it's been a few years ago and I know the gantz manga finish but the anime didn't cover everything of the manga!!