Why is it that most browsers do not support Java applets?

Answer 1

A cursory Google search reveals that the technology it depends on (NPAPI) was dropped as "not sandboxable".
Oracle has some marketing-speak here -- they don't mention why or that their list is short because other browsers did/had to drop NPAPI too.
https://java.com/en/download/faq/chrome.xml
As probably more related to home security, I'd recommend continuing your research at https://superuser.com or stackoverflow.

Answer 2

They have a lot of problems