I think this is mainly caused by hard-core bitcoiners. They resent alts because they believe money going to alts should be going to bitcoin and that bitcoin hasn't made it big because of the presence of alts.
Of course it's a ridiculous attitude. Many people came into the crypto space directly because of alts - for example doge pulled in a lot of people, and a lot of writers are being pulled in by steem.
Finally, the bitcoiner attitude that there should be just one coin is incredibly authoritarian. They claim they are fighting against the hegemony of fiat, but in reality they want to create their own hegemony, and one that is controlled by a small amount of core devs too.
You are mistaken: the core devs don't control anything. Bitcoin users can run whatever variant of the software they choose. It's a voluntary trust decision.
I think its unfair to group all bitcoiners under one umbrella, but definitely bitcoin maximalists are starting many of the problems . I think the majority of people are doing the right thing, just unfortunately the loudest ones are the ones that everyone hears.