Well, certainly I think a part of it was that they could unify on one Chip with the rest of the industry did provide some economies of scale advantages, though rumor has it that Apple has always paid a little bit more for its chips so they could guarantee certain things and dictate as such.
I believe the main driver though, as pointed out in this CNET article https://www.cnet.com/news/four-years-later-why-did-apple-dro... was that IBM could not deliver on a powerful enough powerpc chip that would also meet the other constraints, namely, at this time, one of the biggest markets Apple had was the notebook market, and its sales there were exploding. IBM was unable to deliver a lower thermal envelope for its portable chipsets on top of performance issues.
I think this more than anything else pushed that reality.
I believe the main driver though, as pointed out in this CNET article https://www.cnet.com/news/four-years-later-why-did-apple-dro... was that IBM could not deliver on a powerful enough powerpc chip that would also meet the other constraints, namely, at this time, one of the biggest markets Apple had was the notebook market, and its sales there were exploding. IBM was unable to deliver a lower thermal envelope for its portable chipsets on top of performance issues.
I think this more than anything else pushed that reality.