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.

This phenomenon is not limited to theoretical computer science or academia.

The reason for this is plainly:

• Most users do not think longer than a few seconds about their font choice and its effect. Thus it happens that they think using a comic/quirky/sloppy font is a good idea for a professional talk – even though it’s the typographical equivalent to giving your talk in a clown costume.

• Once they made this choice, this kind of users wants to realise their intention with minimum effort. Hence they browse through their list of fonts – which is most likely the same as the operating system’s default. On many popular operating systems, this will inevitably lead to Comic Sans – even though it’s rather ugly and worn-down for a clown costume.

Now the field influences how “professional” people are. In a field where formal attire is customary, I would expect Comic Sans to be less likely – but I have already witnessed professors for medicine giving talks in tie, suit, and Comic Sans. Also the field’s predisposition to LaTeX may have an effect as it’s a little bit more difficult to use Comic Sans there.

I don't use it in slides, but I did consciously choose it for use for some time. My reason was that because of its design, it is very much easier to distinguish different letters visually that could be confused (i/I/l/1 etc) at first glance, and for computer based work that can be very important. The instant clarity of what I'm looking at was worth it and it was otherwise easy to read. (Spend 3 hours trying to figure out an issue only to find it's a 1 not an I, or an i not an l somewhere ..... I removed the problem by choosing a font for my text editor that excluded the problem entirely)

It was the closest widely available font to otherwise-"standard" fonts like arial, to clearly shape these letters differently enough to be immediately clear.