1) IMO as a community we still haven’t quite figured out the best abstractions for functional UIs. Various FRP and Elm-like libraries are good, but they have drawbacks.
2) Not all frontends have functional APIs, you may be stuck writing a lot of FFI glue code.
3) Front end code often gets pushed to devs with different experience and skill sets, in my experience they’re probably less likely to have functional or hard CS backgrounds.
4) Harder recruiting if all the devs need to be proficient with FP.
I personally don’t find any of these reasons compelling enough to negate all of the benefits of FP.
Answers saying FP isn’t suited for IO or for interaction with users are rhetorically interesting but contradict my experience.
Your comments suggest a frontend perspective. But on the backend there's a ton of code (including business logic, which theoretically should be the most important bit).
I wouldn't be qualified to argue the merits of FP on frontend, but on the backend it really seems to have value.
2) Not all frontends have functional APIs, you may be stuck writing a lot of FFI glue code.
3) Front end code often gets pushed to devs with different experience and skill sets, in my experience they’re probably less likely to have functional or hard CS backgrounds.
4) Harder recruiting if all the devs need to be proficient with FP.
I personally don’t find any of these reasons compelling enough to negate all of the benefits of FP.
Answers saying FP isn’t suited for IO or for interaction with users are rhetorically interesting but contradict my experience.