Acquiring P.A. Semi got them Dan Dobberpuhl and Jim Keller, which laid a good design foundation. However, IMO, I'd lean towards these as the decisive factors today:

1) Apple's financial firepower allowing them to book out SOTA process nodes

2) Apple being less cost-sensitive in their designs vs. Qualcomm or Intel. Since Apple sells devices, they can justify 'expensive' decisions like massive caches that require significantly more die area.

They also had years to keep improving the iPhone chips until they were so good at power efficiency that they could slap it into a laptop.

That’s much better than a decade of development with no product yet.