Everyone trying to extrapolate reasons on why NijiEN might not be doing well and I as someone who has stopped watching them entirely after Zaion got axed. Maybe I can shed some light on this?
1. Ignoring the utter bullshit of Zaion's termination, when the announcement hit it was wave of people being like "this is a good thing I'm so happy, she deserved it" the utter unhinged tumblr style fandom lockstep that a person deserved to be fired over a bunch of innocuous bullshit and celebrating it. I'm sorry, that's some crazy shit and it didn't exactly make me want to engage with the people who tolerated and built that kind of fanbase.
2. The merch is awfully basic, like I had used proxy services to get niji jp merch in the past and whatever is going on with EN it's just not great stuff most of the time, I have income, I like to spend my money, but I'm not going to spend money on the same stuff with stock art tossed on it. Additionally I always have seen merch as the best way to support someone I enjoyed watching because I get a physical item or some such and they get money, it's not me donating it's me getting a product and them profiting. Well Niji's way of handling that seems to be the money goes to anyone but the talent. So why would I bother to buy stock basic bitch merch that doesn't even support the person I like?
Or to put it simply, NijiEN's fans are fucking psychos and I don't want to be associated with them, the merch is trash and it doesn't seem to help the talent at all. It doesn't take a genius to figure out why maybe people aren't throwing money at them. More so when the building next door has managed looks so much more appealing. I'll take the most unhinged chumbud over the average tumblrstyle niji fan any day.