Why changing the subject of a storyline ?

Ok guys I want to learn some knowledge about the anime manga production. The base thing is I watched lots of animes based on games novels mangas. The problem is how are they determining the the ending of a production. Lots of animes has different storylines than their mangas or novels.(Because of this they are ruining the product) I want to know how the comunity determines that "we must change this storyline guys or we are out of funds we must cut the scenes" Who is determining these things. Why are they changing the storyline. What could be the general problem about this issue.

Sometimes it is a budget or deadline issue and they are forced to reach some sort of conclusion in a short amount of time. In other instances they wish to appeal to a larger audience and changes are made for that reason. Having listened in to a lot of panels at cons, those seem to be the primary reasons. Sometimes the creators themselves wish to explore an alternate ending.

İt seems like some lack of prediction. Generaly if you start a work, there must be a coordinator to make a schedule or something but even how big the anime sector is, there are lots of problems. The problems that are occuring are kind of lack of experiences. Sadly :(

You're animating and then an earthquake strikes, production gets behind, sacrifices must be made. First the office intern, then the story.