Contrasts in Number Theory

On a less advanced level than this, people find it very hard to publish expository work. Journals want papers with new results, not papers which explain known results in a way that lets people (students) understand them. I know from experience that there's a lot of demand for the latter, though!

I've started trying to write such things myself [1], because it's almost impossible to find fully-motivated proofs for anything more advanced than first-year undergraduate material. Mathematicians tend to the slick and short, rather than the comprehensible; I have met professors who think their job is done if they take thirty seconds to prove a result, and the student needs ten hours to understand it. This is one of the primary reasons why lectures, rather than textbooks, are still relevant: the lecturer gets a chance to explain rather than just to prove. [2]

Interestingly, in computer science where basic category theory has some traction, the Internet is full of exposition. Methinks there is something of a culture clash between the academics who created category theory, and the compscis who use it.

[1]: http://www.patrickstevens.co.uk/misc/AdjointFunctorTheorems/... [2]: https://www.dpmms.cam.ac.uk/~twk/Lecture.pdf