LaTeX was written and basically frozen in the 80s/early 90s (LaTeX 2e is from 1994). The core ("kernel") of LaTeX cannot be changed as a lot of documents would break. Since then, a very large number of users of varying competence have written ad-hoc packages to extend or customize it. To change the appearance the recommendation often given is to write a .sty file, which is something of a dark art.

ConTeXt is from about a decade later, after some lessons were learned.

More here:

- https://tex.stackexchange.com/questions/223177/what-is-the-m...

- https://tex.stackexchange.com/questions/4987/why-should-i-be...

- https://tex.stackexchange.com/questions/1887/what-typesettin...

- https://tug.org/interviews/hagen.html

- https://tex.stackexchange.com/questions/29979/how-do-latex3-...

- https://tex.stackexchange.com/questions/447566/health-of-con...

My impression: What you get with LaTeX is a larger ecosystem and greater likelihood that someone has already done what you need (in some package or other, though it may be only "close" and figuring out how to improve it may be hard). What you get with ConTeXt, at the cost of having to re-learn everything you learned about LaTeX and not being able to use any of the LaTeX packages, is a more consistently designed interface, a greater understanding of what's going on, more ease of modifying things (instead of the "there's a package for that" phenomenon), and best of all not having to "program" in the TeX macro "language" (you can use Lua).