I recently read mfiano's blog post about Common Lisp. He makes the point that the language attracts loners. To make your project lisp is to ensure no one will want to work in it with you.
He's right. Not just for Common Lisp, either. But I just can't kick lisp. And on the other side of that coin, if I think I will only ever develop a project on a small team, the nimbleness of Lisp helps. I feel like Lisp is custom built for high functioning, high quality, low-head-count teams. I kinda feel like most of my projects fall into this category.
At the end of the day, it's just more fun. And I am doing this stuff in my spare time.