Middle click paste is a very useful feature for a lot of people, but new linux users are not those people.
I personally switched two years ago, and got several people I know to switch too. Everyone I know who switched (including me) was confused by middle click paste.
It’s a hard to intuitively understand action (took me several months until I understood it took the selection for some reason) that is very easy to trigger accidentally, and that duplicates existing functionality.
The people who like it already know it exists, and could just toggle it on.
Of course, on distros not aimed at beginners, like say, debian, it should remain the default.
I was under the assumption it was so you could copy/paste in the terminal. Highlight to copy, middle click to paste. Ctrl+c / ctrl+v are used by other things in the terminal already.
I think that is originally how it was used because GUI apps were less prevalent. But there is also Ctrl+Shift+C/V for terminal copy paste. And in Vi you have Y for yank and p and P for put. So many clipboards😱