There are two obvious reasons why Apple might wait before it adopts 5G cellular.
One, 5G barely exists yet.
Two, Apple had to shift gears drastically when its plans to use Intel 5G modems fizzled, reportedly forcing the company to settle all its lawsuits with Qualcomm so Apple could use Qualcomm as a supplier instead. That ended up killing Intel’s modem business, which Apple wound up buying in the end. Serious corporate drama.