Could some Xcode uers explain to me why AppCode (IntelliJ IDE) did not take off as an Xcode replacement? I recently had to do some iOS work, knew that Xcode sucks, wanted to try AppCode, only to learn that it was discontinued because of lack of interest.

AppCode was unfortunately extremely limited. Lagged behind Swift versions. No editor for Storyboards/XIBs back when they were ubiquitous, and no preview for SwiftUI. Debugging app extensions or anything that isn't the standard "iOS app" was generally flaky or outright not supported.

There is a lot of interest as IntelliJ is a good IDE, but AppCode just wasn't reaching the bare minimum required to have me not use or worry about Xcode.

My problem with AppCode was that the syntax of Swift was still undergoing rapid development, and AppCode lagged too far behind. So AppCode wouldn't understand parts of my code unless I avoided the latest (often very valuable) additions to the language.

When it did understand my code, it was much better than Xcode for navigating and refactoring.

Also, AppCode couldn't edit or even display XIBs so I often had to keep Xcode running alongside it anyway to edit XIBs and make connections between XIB and code, and that was a hassle.

I'm only guessing and I'm not an xcode user but my general observation is that Apple users have a culture of using whatever Apple gives them, whereas in the Windows world the culture is to go out and look for your favorite third party option.

App code was a really great IDE, Especially if you were developing server-side Swift. I wish JetBrains would've open sourced the plugins so it could carry on

If I remember correctly, you still needed Xcode to actually compile and run the code, because of restrictions imposed by Apple.