cost. it is stupidly expensive to include any sort of commercial MCU core(s) in your chip. Ancedata: ARM won't even talk with small ASIC fabless companies, even if they are willing to shell out big upfront costs and royalties per chip that ARM demands. Having free or affordable alternaties is a great driving force for the industry.
> ARM won't even talk with small ASIC fabless companies, even if they are willing to shell out big upfront costs and royalties per chip that ARM demands
[Citation needed] There's literally 0 upfront costs for a Cortex-M3 [0]