Why doesn't Intel include an Atom core on their CPUs as a next level speed step, like ARMs big.little? It has to be much easier to keep both the high and low power parts the same architecture.
That would probably be less power efficient than just running a single core alone at reduced clock.
Which is why the Atom was discontinued, IIRC: the mainline chips power efficiency had been reduced enough to make the Atom much less viable.
Atom hasn't been discontinued; low-end "Celeron" and "Pentium" chips are actually Atoms.
Both Pentium and Celeron are used for standard mainstream dual-core chips and for Atom chips (eg. J4205, Celeron G3900, Pentium G4400)... so in a way they are Intel's trash-can brand.
I think you meant the mainline's efficiency increased. :)