It's probably because there isn't a tribal majority in that state.
Assamiyas, Tripuris( Bengalis ) it's a given because they are Indic anyway.
Majority of Arunachalees are from local tribes and not from outside states, very few outsiders live there. I've lived there for 3 years and the localites there are extremely patriotic , unlike Nagas , the people there constantly live in fear because of chinese expansionism into their home state. There are many tibetan refugees in villages like shergaon, dirang dzong who tell about the horrors of tibetan genocide to the local people , the locals of Arunachal know about the horrors of communist occupation and this makes them resent anything which is related to china. Another reason is that a very large chunk of arunachalee people work for the army and BRO. They are employed as Soldiers, policemen, CPLs, porters, teachers in army run schools, etc. There have been many cases where PLA troops came to border villages to spread propaganda but the locals immediately booted them out and called the army/ITBP to handle the situation. They even bring out their hunting rifles to scare away PLA troops