As someone that got introduced to it in 1996, and was a fan of XEmacs variant, still remembers enough keybindings and Elisp, supported beyond plain syntax highlighting was very much hit and miss, even nowadays.
If the old emacs approach is so much better, why is emacs switching to using LSP?
It isn't switching. It added support for it. Because that's what emacs does; it's the kitchen sink. Its mission in life is to do absolutely everything. That includes speaking shitty protocols.
But the effort to do so was then entirely duplicated for other editors. LSP turns an N*M effort into an N+M effort, the upshot of which being far more complete coverage for both editors and languages.
I have been an emacs user for over a decade now, and if you think that language “support” was as good before LSP, I have a bridge to sell you.