There's a good summary of why Cardno uses Haskell here - https://medium.com/@cardano.foundation/why-cardano-chose-has... - functional programming seems to have been a key influence in how Cardano's approach to smart contracts is built, avoiding centralized state - some insights in that here https://sundaeswap.finance/posts/concurrency-state-cardano

Potentially Cardano could become one of the most significant uses of Haskell in the wild, if it gets traction of course