I believe the specific reason (as far as Apple disclosed) was that lower performance / higher efficiency PowerPC CPUs just weren't on IBM's roadmap and whatever

I believe the specific reason (as far as Apple disclosed) was that lower performance / higher efficiency PowerPC CPUs just weren't on IBM's roadmap and whatever quantity of CPUs Apple was buying and/or willing to commit to buying wasn't enough for IBM to consider it. Intel was focusing on power efficiency after the whole Netburst disaster.