Why does Bella not get turn into a Vampire in the movie twilight?

because edward is scared that Bella will forget that shes in love with him.

Though this answer is correct the biggest reason is the fact that being immortal is not as glamorous as it seems, from Edwards point of view Bella being human would be the best thing, and they can continue there lifes while Bella is mortal (gets the end of the movie when he says "Isn't enough just to have a long happy life with me?"). He doesn't want her to jump in to something that isn't right. However in the other books Bella fights for her right to make her own decisons on whether she will be best mortal or immortal.