Because the language of the week changes often, and learning can be done by solving the problems of today instead of rewriting software into a version that will never be used. I mean... who still uses all the rewrites to ruby?
Even emacs was rewritten to rust ( https://github.com/remacs/remacs ), many hours were spent, and the last actual code commit was 5 years ago.... why not spend that time by making the "normal" emacs better? Or make something new in rust?
It's usually to help teach them the other language.
The rewrite part makes it easier for observers to learn and compare/contrast techniques in different environments for themselves.
Why this would ever be criticised, I can't imagine.