The reply?:
Memory
The Node or Python runtimes were meant to be run by individual people on their own servers. They were never intended to be run in a multi-tenant environment with thousands of other people’s code and strict memory requirements. A basic Node Lambda running no real code consumes 35 MB of memory. When you can share the runtime between all of the Isolates as we do, that drops to around 3 MB.
and for startup time of a process:
Because Workers don’t have to start a process, Isolates start in 5 milliseconds, a duration which is imperceptible. Isolates similarly scale and deploy just as quickly, entirely eliminating this issue with existing Serverless technologies.
So 3MB is clearly much more memory needed than the KBs from the BEAM, and 5ms seems an eternity in the BEAM land 
as pointed out in the Elixir forum by @malaire: