Because "fake news" has a very clear meaning that should be apparent to anyone who knows what the word "fake" means. Where do you use the word "fake"? You use it in places where something that is known to be false by the originator has the appearance of truth.
You do realize that would not include most conspiracy theories, right? Sure, some are created and perpetuated because it fits somebody's agenda but most of the unpopular ones are simply people seeing patterns that aren't there, explanations as cover-ups, ridicule as naivity or complicity resulting in some bizarre theory that doesn't make any sense or serve any recognizable purpose.
It becomes fake news when they report bullshit without fact-checking because it sounds truthy and if true, supports their point. And this is what happens every time. Supposed chemtrail planes which are actually full of tanks used to create different weight distributions for testing, for example.