It might have to do with the way the default generics for aesons fromjson and tojson instances for record types are defined? Or maybe the lack of incremental streaming read write support in the aeson data model? Or maybe they wanted better date or integer support? It'd be pretty easy to ask don or Lennart I'd imagine.

I have to say that I definitely enjoy using haskell when faced with dirty complicated data, which I'm told happens a lot in larger organizations.