Because these days this is my bar for a language that is "nice for building web applications". I've gotten so much mileage out of Clojure+ClojureScript just because of this, it's not even funny.
If you want statically checked types, I'd probably say that Scala is better at the server+client game (i.e. when you want to have both). Of course, Scala has its own drawbacks wrt. Haskell: lack of typed effects is a big one for me.
(Just for context: I work on a website+SPA written entirely in Scala which has been around for years and years. I also have quite a lot of experience in Haskell.)
There are libraries like ZIO, Cats Effect or Monix. Give them a try! Some people might even say that for example ZIO is even better than Haskell's IO.
That truly is the singular reason that I still prefer Haskell over Scala. I can get over syntax awkwardness, etc. etc. The impure-in-pure is... difficult when you can't just grep for unsafe*, etc.
EDIT: Fwiw, ZIO is definitely better than Haskell's IO. Better than RIO? Perhaps. Is it better than polysemy, tho? I don't think so. Btw, I know about polysemy's issues as well... hopefully lexi-lambda can get her GHC runtime changes merged so that we can have a true "free" effect system backed by a tailored runtime. Interestingly, Project Loom is also heading in a similar direction (first class continuations) on the JVM. Interesting times!
Especially since you can autogenerate Elm decoders based on Haskell types to reduce boilerplate and duplication between frontend and backend, while mostly retaining a strong sense of type safety.
You can also use Elm as an introduction language to new developers, without all of the complexity of haskell’s higher order abstractions, and then introduce them to haskell when they’re comfortable with elm.
(I did this with Servant as a backend, but IHP is wonderful as well!)