PIC extreme low power (xlp) is ridiculously low - at least an order of magnitude lower sleep current (9nA) than ARM equivalents. They also have very low uA/Mhz,

PIC extreme low power (xlp) is ridiculously low - at least an order of magnitude lower sleep current (9nA) than ARM equivalents. They also have very low uA/Mhz, but some ARM chips are approaching or exceeding that (eg SAM L). There's also something to be said for simplicity and ease of hardware development, as opposed to a race to the bottom on price. Do you necessarily need all that ARM grunt? 8 bit micros tend to need very little support circuitry and can have very low pin counts - eg the PIC10F322 is 6 pin, 20nA sleep current and comes in a DFN package. There are also applications where having a DIP package chip is useful (easy replacement, for example).