I don't see a reason to write the genetic algorithm in Lisp, but for the code that's being evolved it seems like the logical choice due to its minimal syntax. I'm working on a PHP program for evolving programs, and the evolved programs are in a lisp-like dialect made for just that purpose.

IIRC it's something to do with the fact that lisp can easily be represented as a tree and so crossover is very simple; just swap branches. I don't know lisp so I'm just regurgitating info I picked up at university.

One problem with using that technique is that it's has been patented, along with derivatives of it, for quite a while.