Why do theoreticians in CS use multiple-letter variables?
Because we're computer scientists as well as mathematicians, and from the computer science side of the world we've learned the same thing the software engineers have learned about why not to use single-letter names for things other than the most local of variables: longer and more semantically meaningful names are easier to remember and keep straight, easier for others to read and understand, and using them helps avoid certain common errors such as using $k$ to mean three different things in overlapping parts of a paper.

(Actually, I suspect that for the specific case you had in mind, names of complexity classes, this reason is historically inaccurate. But I still believe it's a good reason for using longer names.)