Lisps make great configuration languages because of how dynamic they are. It's like Javascript on steroids. Redefine core symbols. Modify a syntax form and reevaluate all of your code so that it does one thing in one environment and something else in a different one. When building large software in a team this can be a big drag; learning some weird syntax form that a coworker merged in that you didn't review can be a headache and when everyone makes their own design patterns, the codebase becomes a mess.

But when you're just configuring software this heavily dynamic nature becomes a strength. I just need to build the abstractions in my config that I need, nobody else. It's easy to poke around in and spend a few minutes writing and iterating on code to get the config I want.

I agree that the prefix notation and the soup of parentheses give it a higher learning curve than something like Lua which is easily understandable if you have experience in pretty much any modern programming language. But it's not the mathematical/computational purity that makes Lisps appealing, it's their highly dynamic nature.