I guess, it would be kinda harder to adapt Haskell to the client-side-only interactive web (but of course, people are trying [1]). Nowadays the availability in the web directly impacts its acceptance so it is not a separate concern.
That's kind of outweighed by the more relevant fact that the people who had the time and inclination to create the software preferred one set of tools, and the preferences of some after-the-fact passerby would obviously not factor into this kind of decision.
[1] https://wiki.haskell.org/The_JavaScript_Problem#Haskell_-.3E...