Why other northeastern state are not rich as sikkim

I am a mainlander so I don't know much about northeastern state but why can't other northeastern state replicate the success of sikkim. In sikkim all 75% of working population employeeed in tourism sector directly or indirectly.All northeastern state have less population like sikkim except assam. Population density is also same. Pretty sure there would be alot of good natural places to visit in other state also. Population density is less so there must be alot of agricultural land for organic farming