Because depending on your programming language it can be anywhere from annoying up to impossible to load and utilize a shared library. IO (and especially HTTP)

Because depending on your programming language it can be anywhere from annoying up to impossible to load and utilize a shared library. IO (and especially HTTP) however are easily available in most environments. Also languages servers might often be written in the language-that-they-care-for themselves (e.g. the C# language server in C#), because that's the environment where most of the tooling for that language is available. If you need everything in C then you need to rewrite the tooling. Or you just build a C library which then internally does RPC to the non-C process, but in this case you made the language-service an implementation detail for one language.