Python has been massive since the 2000s. When AI rolled around, it was already there, a bunch of people knew it, and it was Good Enough (tm).

I think its the other way around. Python became the language of choice for AI because it was already popular. Lots of things made it popular: use for systems management scripts, web apps (Django, especially), then numerical stuff,...

I think the reason is that it is easy to learn enough to get things done, but it is very flexible, very readable, and once the ecosystem started gaining momentum (which it clearly had by the time of the XKCD cartoon) that became an advantage too.