why is that so that some manga aren't finished even afer Years?

Mainly coz they are either licenced forcing them to drop it or the scanlators group disbanded or just don't feel like continuing :/

It depends on the situation. Most of them just don't get enough fans who want more of the manga, so it becomes unable continue.

*Oh and also the author might had died (RIP) or just got health issues and won't continue their work.

thanks for telling me this.