Why is Nya often not considered as part of the main team by the fandom?

I've seen a lot of people mostly only care about the original 4/5 ninjas (Kai, Jay, Zane, Cole, and Lloyd), and even the show sometimes (at least pre-Dragons Rising). I understand she wasn't a ninja until Possession, but I still consider her part of the ninja team, instead of "an extra ninja" like I've seen a lot of people treating her like. pretty sure its because - she wasn't part of the original 4 (Kai, Jay, Zane, Cole) - she isn't the master of one of the elements of creation (fire,...