I think Ninjago is one if those things where you either grow super attached to it if you were right from the beginning, or if you were just casually into the theme, you quickly grow out of it. I have started watching the series pretty late into its run, when it was already at season five, so that might explain my lack of dedication to it.
I think the series not growing and maturing with its audience is a big turn away factor for many people (myself included). Despite being around for 10+ years, the characters feel like they haven’t grown whatsoever, being almost exactly the same as the first time we met them. For a line that has ran for so long, this is pretty embarrassing.
Honestly, in it’s current state, the Ninjago TV Show doesn’t hold that much value for anyone above the age of 12, and those older than that who still watch it mostly do just for the sake of nostalgia. My younger brothers are 10 and 12 respectively, and while they had a brief Ninjago phase a while ago, they’ve long since grown out of it, and I am willing to bet most other kids within that age range did as well. If the writers would have made an effort to make the series more mature, say, the way Avatar the Last Airbender did, I am willing to bet things would have been vastly different. But for the most part, they haven’t treated the series anymore than your average series that was created just to sell toys.
Also, from my experience as someone who’s been involved in the Ninjago community for a while, I can safely say that they are the most imature, childish and cringe worthy people I have ever met online. I really hate saying it, but that’s how my experience was, unfortunately (and I’m sure many other people who are Ninjago fans themselves would agree).