Alternative possibility (that I just made up on the spot): the people who get to choose what language to work in, ignoring the popularity of the language (and therefore how easy they are to replace) are already in very senior positions. More junior people do not get the opportunity to work in functional languages.
It's half of that. On the chart "Salary and Experience by Language", people doing functional languages have much more experience.
The other half is location. There are only few functional programming jobs, highly skewed toward the big cities with higher salary and costs of living.
