That's like asking what LSP is missing that visual studio provides.
Slime is a full-blown IDE that just happens to be implemented in elisp.
That being said, LSP doesn't do anything for providing indentation rules to editors; editors not named "emacs" are at best poor at indenting lisp code, and many modern editors do not provide pluggable indentation. Because of the fewer distinct visual blocks in lisp, properly indented code is important for readability.
I think the main reason an LSP hasn't been implemented for lisp is that nobody that has the skills to make one has any use for one.
Slime is a full-blown IDE that just happens to be implemented in elisp.
That being said, LSP doesn't do anything for providing indentation rules to editors; editors not named "emacs" are at best poor at indenting lisp code, and many modern editors do not provide pluggable indentation. Because of the fewer distinct visual blocks in lisp, properly indented code is important for readability.
I think the main reason an LSP hasn't been implemented for lisp is that nobody that has the skills to make one has any use for one.