I don't actually know but my assumption is that that they're working with a very old codebase based on the "bespoke parsing and plugins for every language" paradigm that served them well for decades. Meanwhile eg VSCode is using the "language server with treesitter and queryable compiler genetically integrated over a standard API" model that's only recently become widespread.
When I first learned about LSPs it was immediately clear to me they would run circles around the "traditional" IDEs. I'd given up on using IDEs because I found them too finicky and error prone, but LSPs have been a total game changer.