Why do theoreticians in CS use multiple-letter variables? This is a "dual" question of a popular post on math.se.

Some mathematical objects in computational complexity theory have multiple-letter names.   Complexity classes such as $\mathbf{BPP}$ have an established multiple-letter name.  So do some computational problems, although the naming convension seems to vary.

I am happy with names such as $\mathbf{BPP}$; however, when I write $\mathbf{PromiseBPP}$ on paper or blackboard over and over, I wish its name were more concise (in fact, some authors prefer to write it as $\mathbf{prBPP}$).
In a popular textbook in complexity, a polynomial that appears in the chapter on algebraic complexity of the book is named as $\mathtt{EXACTLY-ONCE}$, which I do not think is a suitable name to write by hand.

Considering the arguments in the original post on math.se, the pros of shorter names seem to apply more to TCS than those of longer names.  After all, what theoreticians do is to work on paper and blackboards, not to write large and complex software with editors that do auto-completion.

Some early literature takes different approach to names.  In Cinderella Book, computational problems have concise names such as $L_{\mathrm h}$ (which stands for the Hamiltonian path problem).

Thus the question is: why do theoreticians in CS use multiple-letter variables?  Or do we have to give long names to objects we are working on?
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.)