The case for Greek mythology is because it was very well recorded it is well known. Also it had big impact on western literature, etc. The prestige of ancient Greece and Rome which imitated Greeks must not be unnoticed in this context. Norse mythology also was “reasonably” well recorded (far less than Greek) but it doesn’t have cultural impact and importance of Greek mythology for various historic reasons. It gained some foot ground via film tho but it can’t be on the level of Greek one. Slavic mythology on the other hand had to be mainly taken from surviving folklore recorded mainly in 19th century from oral sources and vague records before that. That being said i don’t want to say any of them is superior or inferior. I do agree Slavic mythology should get more “publicity” and attention.