Because it's developed by JetBrains (with Google contributions), a company whose main business is writing really good IDEs. Apple on contrary is a hardware company that happens to build software. If they had delegated the XCode development to JetBrains, we would have had a great IDE for macOS/iOS development too. AppCode was damn good with zero support from Apple side, and despite the fact that JetBrains always needed to catch-up with Apple's breaking changes.