The problem is that cabal-install isn't included in Ubuntu's package manager, so it would require manually installing cabal-install. The maintainers are pretty conservative when it comes to adding more steps to the server setup because it makes things more complicated and also makes it harder for others to set up their own instances. If you write up some very explicit steps and maybe even provide a patch for the setup script, they might be willing to include it. I was going to do this, but I never worked up the motivation to push it hard enough.
There are a couple alternatives. One is to just include the libraries you need with your bot. I have done this in past contests. It was awkward, but could be made to work. As for the other alternative that I'm going to be trying this time, I think I'd rather explain it on IRC.