Your point about "embrac[ing] the Unix philosophy" is a good one. Probably not the biggest factor for mainstream adoption, but significant nonetheless. While there are some Lisps now that that do this well (particularly those that compile to C, such as Chicken Scheme), it has a big historical precedent.

I've been wondering how Lisp would look if it were redesigned from the ground up to include Unix's philosophy, and I'm beginning to think that Erlang would be a good candidate. (With a healthy dose of Prolog as well.)