A lot of it is the nature of the business side of creativity - you want to put out something people want to buy. Even writers will get swayed by something they've seen that was good and successful.
Like somebody pitches a story where puppets are a real class of people in the world - sorta like Toons in Who Framed Roger Rabbit. So the basic pitch is that three of these puppets, a Badger Berzerk, Sandy Salamander and Gila the Monster get fired from the kid's show where they were bit players for getting caught smoking on camera. Broke and needing cash to keep out of the trash bin, they take a job from a local criminal to kidnap a rich man's daughter for ransom. Since she loves puppets, it's easy for them to lure her away, but while they keep her on ice until the real crooks to pick her up, they end up falling for her. So, they don't want to turn her over and go on the lam with the kid. They want to give her back to her parents, but they don't want to go to jail, and because they come up with a super complicated plan to give her back, it all goes wrong and they lose her in the middle of the city.
So let's call it STITCHHEADS and say that pitch gets approved for development. Everyone is like, it's THREE GODFATHERS meets ROGER RABBIT but with muppets. It's an easy concept to grasp.
Then I go see PLANET OF THE APES, and I'm thinking... what if this was a story about an uprising of puppets who are tired of being kept down by the human race? Or someone else sees a zombie movie that hits big and wonders - maybe the puppets could be monsters. Like CHUCKY meets GREMLINS, instead, and it's really a horror movie? And somewhere in there the original idea gets completely buried because we can't develop these stories without the entire world of stories around us influencing it.
In some cases, it's a good thing. GREMLINS, BEETLEJUICE and THE MASK were all originally gory horror movies and the moment someone said "what if it was really a comedy?" the movies became what we saw on the screen.