Apparently the quality assurance of Skylake processors was so bad Apple decided it had to make the leap. François Piednoël spent 20 years working at Intel as both a Principal Engineer and Performance Architect.
https://medium.com/pcmag-access/former-intel-engineer-explains-why-apple-switched-to-arm-deba86e560b1
https://medium.com/pcmag-access/former-intel-engineer-explains-why-apple-switched-to-arm-deba86e560b1