The immediate reason was lack of support in LLVM, but I suspect the reason why it wasn't re-implemented was that the demand wasn't enough to justify the significant amount of work that would be needed to get it working a) with LLVM and b) in the locked-down environment of iOS.
The speed boost from LLVM, SSDs, the "thin" nature of iOS apps, and the bump in processor performance was, per my intuition, enough that a typical "full" build today can handily beat a typical "Fix-and-Continue" build from a few years ago (I use the term "full" to mean a build that updates one or two compilation units and re-links).
It was a nifty feature and I wish they'd bring it back, but I'm pretty sure the XCode team has lower-hanging fruit (static analysis, code navigation, refactoring).
The speed boost from LLVM, SSDs, the "thin" nature of iOS apps, and the bump in processor performance was, per my intuition, enough that a typical "full" build today can handily beat a typical "Fix-and-Continue" build from a few years ago (I use the term "full" to mean a build that updates one or two compilation units and re-links).
It was a nifty feature and I wish they'd bring it back, but I'm pretty sure the XCode team has lower-hanging fruit (static analysis, code navigation, refactoring).