What I don’t understand is: what features of Python encourage its use in embedded systems in the first place? Why not F# or Elixir?

Python is everywhere in education, F# and Elixir are niche languages.

If you want to use F#, you might get lucky with Meadow boards, yet their focus is mostly getting C# to run on their runtime.

Lots of developers already know python, and if you don't it's easy to get rolling.

For pyboard, it's STM32F4 with about 200Kb of RAM. AFAIK, F# and Elixir target VMs (CLR and BEAM) and it seems unlikely that those VMs plus your code will fit on such a chip.

Ease of use and familiarity to people who aren't principally interested in programming. These are people who want to make things work using microcontrollers.