Its funny because I've found the opposite; the fact that the smaller community (and lets face it - most FP langs have a much smaller community than needed to get the scale you are after) the more it needs to piggy back on the army of engineers in the mainstream language target and stick to only its value add as a technology. There were web frameworks in F# that were written from scratch, but fundamentally ASP.NET is quite good and fast enough at least from my evaluations and these frameworks are now out of favor.

I personally don't have much issues using ASP.NET directly from F# (don't bother with Giraffe or anything like that); its not too hard. The routing layer is only a few classes/functions anyway - it isn't the majority of your program. I guess maybe I'm in a different position to you - I find for the few bits of ASP.NET config it isn't worth switching to C#. A few rules you need to follow but I would say that is a lot less than the Clojure learning curve and you get static typing still - that is just my opinion however.

My experience is that both languages have their pros/cons and tbh both have their challenges w.r.t adoption. My personal view is most of these challenges are not technical. The issues we complain about with the language as developers are IMO minor (i.e. things are still possible or decent compared to some other languages) and probably aren't the major factor in the wide scale technology choice away from FP in general.

For better or worse developers are often "languages takers" not "language deciders" in most big shops and this is where most of the jobs are - its only in niche elements/problem spaces (i.e small shops, dedicated niche teams, etc) where niche technologies can be worth it. This means not a lot of jobs; but when there is a job it can be higher paying. Big employers drive the market for technologies though when it comes to large scale usage and they want commodity fungible developers (i.e. resources), and are happy using a mediocre tech that gets the job done to do it. Unless there is a "killer app" where the language must be used to unlock the value most managements will stay clear of better tech - predictable, boring, commodity tech that I can get anyone in for (contractor, offshore person, etc) wins for them every time especially for the MBA/product manager/etc types. In the current economic climate developers are trying to meet the jobs that are being advertised as well (higher interest rates, less startup's, etc etc).