At the time they thought it would be too slow. In retrospect it would've been fine, I think. IOKit is a pretty strange subset of C++, but at least it doesn't use all those templates.
That was my immediate thought, speed issues. MacOS isn’t exactly an I/O speed king as it is. I suspect if it had made it in it may have been removed by now.
It was in NeXTSTEP. It was taken out because of performance concerns that turned out to be entirely unfounded. Alas, when they figured out that this was the case it was too late.
A similar example is CoreFoundation. It was a C implementation of the OPENSTEP Foundation. To this day, even really smart, capable and knowledgeable engineers believe that CoreFoundation is faster than the pure Objective-C Foundation. It is not. It is significantly slower. Often several times slower.
(It is of course very slightly faster than the "new" Objective-C Foundation that was implemented on top of CoreFoundation).
Another example is Swift. People believe it to be generally faster than Objective-C. Again, this is not the case. Swift code tends to be significantly slower than comparable Objective-C code.
