I suppose there are 2 reasons for it: 1. Popular languages would have lot more developers so lesser salary (Supply-Demand) 2. Since the newer devs have starte

I suppose there are 2 reasons for it: 1. Popular languages would have lot more developers so lesser salary (Supply-Demand) 2. Since the newer devs have started their careers when the newer languages have started to get mainstream, more senior devs are the ones who remain with the niche languages.