Why do many anime don't finish a manga's story and are given their own ending?

[Star_Of_Hope]

So, what do you guys think about it?
Is it because it takes too many episodes to cover the entire story? Or is it something else? I am talking about completed manga adaptations here

 

[Blackangel]

lol may be because they don't have the budget
or it can be related to their own story-lines requiring more and more episodes lol