There are multiple reasons, but the single biggest reason is: Apple A customer that is willing and able to ramp to huge volume a small size chip (important for

There are multiple reasons, but the single biggest reason is: Apple A customer that is willing and able to ramp to huge volume a small size chip (important for good yield), and even willing to ramp with relatively low yield (due to high iPhone margins). Time to volume production is generally gated by yield for foundries, and Apple provides an intermediate target with the small mobile chips.