Plenty of microcontrollers have a single-digit number of Cortex-M cores and memory/flash counted in the megabytes. It'll be decades until that market reaches the multi-gigabyte point, so why bother wasting a whole bunch of memory on 64-bit pointers?
I'm not quite sure why you'd want to run Erlang on it, but the hardware exists.
https://nerves-project.org/#features has a decent pitch for why. (Most of the features listed here aren't features of Nerves-the-Elixir-IoT-runtime-codebase per se, but rather benefits of Nerves-the-toolchain enabling you to easily build lean, embedded Erlang [on Linux] firmware images.)
Erlang is invented before IoT was a thing to facilitate distributed computing for telecommunication in a highly reliable manner. It makes perfect sense to adapt it for driving fleets of cheap IoT devices.