It's only because any material that isnt conceived in the format of the original iteration of the franchise is usually just Extended Universe stuff, and the reason all this EU stuff isnt considered canon is because, while they provide a vast amount of information that fans may or may not agree with, a lot of it can be contradictory. Consider how massive the Alien/Predator EU is after 30+ years - would you really want to catalogue all of that into something that makes even the least amount of sense possible?

Of course, you could always just nit-pick your favorite parts and say THOSE are canon, but how is that fair to the entirety or the fandom? The only way you could possibly do this correctly would be to take these previously non-canon tidbits and MAKE them canon through the original media. A good example of this would be Predators - previously, only the non-canon game, Concrete Jungle, had a name for the original '87 Predator, which was Jungle Hunter. When Predators came around as a legit sequel to the main franchise, it included the fact that the Jungle Hunters is indeed what this particular clan of Predators was called.

That doesnt mean it's the same for every franchise, where some different-media titles are considered canon and same-media titles non-canon. Another big franchise, Fallout, is a good example; they started out with games, so games are the main canon, while any movies, comics, books - unless stated otherwise - are non-canon. And even then, another Fallout Spin-Off (I think it was Brotherhood of Steel) is a game and it's considered non-canon.

Even going back to Aliens/Predators, Colonial Marines is a game and it's considered canon. AVP is a film and it basically doesn't exist. There are always exceptions to rules.

@stevethepwny is that true
if so where did you get that info from ?