Habit, confirmation bias, sunk cost fallacy.
Namely: people have gotten a lot of battle scars by working with what pays their bills -- PHP, Ruby, Python, C#, Java -- and they refuse to look at an alternative because that would render their huge time and energy investment moot (in their eyes at least; I don't see why this has to be the case but plenty of people have been adamant about this without giving an explanation).
I've only become a better programmer since I adopted Elixir but I never stopped using other languages.
All of that plus what PG calls "the middlebrow dismissal" are the main reasons IMO. People are just too set in their ways.