The vast majority of packages that we are depending on for Python are highly dependent on GCC for compilation and installation. I just completed an installation of 180+ Python modules, with most of it source code being compiled on AIX.
Before we switched to GCC, we had a constant issue of updating the make file to be compatible with AIX's make; updating C files to be compatible with xlC, and so on forth. With the switch to GCC, we only encountered 3 individual issues, all with tickets opened to the respective developers to improve compatibility.
We just simply do not have the time to figure everything out on AIX using xlC.