Academia - Why are the majority of questions here related to either CS or mathematics?

As Aeismail points out in their answer, the Stackexchange network grew out of the StackOverflow site - which is for programming queries. Therefore, people who program in their daily lives, i.e. people in STEM subjects, will be more likely to think of coming here.

But it's perhaps also worth noting that a huge number of questions that are closed for being off-topic are from computer scientists; for some reason they seem to assume that "academia.stackexchange" means a place to ask academic computer science questions. This has always baffled me. Maybe it's for the same reason?

While there is an actual prevalence of people from those fields here (due to reasons elaborated in other answers), this even gets emphasised due to the fact that they are somewhat peculiar due to their subject of research and history, for example:

• The research process in many subfields of mathematics and computer science can be quite different from, say, an experimental field.
• Publications in mathematics are put to special scrutiny (for a reason).
• Computer science has a mostly unique tradition to publish at conferences.

Compared to this, many other scientific fields are rather homogeneous – they form the default backdrop, against which the above peculiarities can be seen. Therefore, for mathematicians and computer scientists, their field is more likely to be a relevant factor to mention in a post – and thus more visible.

Note though, that there are other fields with similar peculiarities such as law, which you will hardly ever read about here, because they are indeed underrepresented.

As its name suggests, the Stack Exchange network grew out of CS-based websites, so there is likely to be a lot more questions from scientists than from the humanities, because that reflects the user base.

It’s also worth noting that there aren’t nearly as many resources geared toward STEM faculty: most of the books I’ve seen addressed to faculty have been written from the viewpoint of someone who is working in the humanities rather than the sciences.