The answer mostly is: Because I liked Haskell and knew it well; and I and a colleague wanted wanted to rewrite the job scheduling system and just picked it. But as soon as we started on the project we were extremely productive and were able to launch a proof of concept within days. That really got us excited and showed us a lot of potential in continuing with this approach. Especially things like QuickCheck and testing of pure functions allowed us to write code quickly and correctly with ease.
Also most developers (and also leadership) in our team at the time already followed 1 or maybe even multiple university courses involving Haskell so we had quite a bit of 'hidden' expertise.
For the next project; the API gateway; I honestly felt it was an even better fit. Haskell's Web Application Interface and its ecosystem of middleware is top notch and makes it really easy to write very performant web proxies and servers. Though if I'd have to make the same decision today I'd probably pick Go over Haskell given Go has an even wider ecosystem of middleware; and has a way stronger story surrounding cryptography in which Haskell is severely lacking