Why do theoretical computer science people use Comic Sans in their slides?
There's no deeper reason behind it. I think people just like it, particular its informal feel.

Theoretical computer science is a pretty informal and laid-back field. Although in many fields Comic Sans would be considered unprofessional, the standards for "unprofessional" in TCS are much more relaxed. For example, I've observed that it's far more common for professors and students to be on a first name basis with people in the TCS world, and the tone in which papers are written are much less formal. It is typical for times problems to be described with little narratives or jokes, and often problems retain names based on how they were originally presented. For example, there’s the Handshake Lemma, the Traveling Salesman Problem, and the Arthur-Merlin Protocols. That’s not to say that informal names and presentations of theorems don’t exist in other fields, but in TCS I would go as far as to say it is typical.

Heck, in his recent landmark paper on Graph Isomorphism, Laszlo Babai wrote something that could very reasonably be interpreted as a sex joke. In many fields innuendoes such as the one found in his paper would be considered wildly inappropriate. I don't know if the sex joke was intentional, but the line (although it's been noticed by many people) hasn't seems to cause any blowback or embarrassment.