New languages are coming out all the time. For every one like Python that proves to have serious staying power, there are dozens of others that slowly stagnate. It doesn't typically make sense to bet your business on a new unproven language; you want to use something that's already been around for awhile, clearly has staying power, and has a decent community built up around it. Lisp has fit that bill for decades now. At the time of Viaweb's founding, Python was not there yet. The closest thing that was would've been Perl.
Python didn't even have lambda/map/filter/reduce until '94.
Lisp was already Lisp.
Python's lexical closures are still crippled even today, no?
Since Python 3 added the nonlocal keyword, they seem to be fine, although I haven't examined them too closely.