Apple had two technical needs that GCC could not reasonably satisfy. One was to compile Core Image filters. When you make a chain of filters, they treat it as a single complicated filter which they JIT compile and optimize for your GPU or CPU (I believe they decide at run time which to target based on what will be best for the particular complex filter you have defined).
Another technical need they had was for a compiler system that could be easily and tightly integrated with other tools, such as IDEs and debuggers.
Either of the above would have required extensive modifications to GCC, and they would not have been able to get those modifications into the upstream. The result would be Apple would have to maintain a fork of GCC. They'd be spending a lot of effort porting things from upstream into their fork.
Another technical need they had was for a compiler system that could be easily and tightly integrated with other tools, such as IDEs and debuggers.
Either of the above would have required extensive modifications to GCC, and they would not have been able to get those modifications into the upstream. The result would be Apple would have to maintain a fork of GCC. They'd be spending a lot of effort porting things from upstream into their fork.