Those little cores get a lot more benefit out of special case instructions.

It's less that, and more that in high-performance-at-low-power realms, density (and CISC) is good - it means big complex operations can be hidden behind single instructions instead of needing multiple instructions and powering all of that hardware for multiple cycles.

The Atom needs instructions that make bigger, more complex operations simpler and lower powered, and with transistors the size they are these days they have ample silicon space to burn on these types of accelerators.