Since they were on a boat with a Raspberry Pi and not much battery power or internet, in addition to execution speed and portability, they were probably also concerned with:
- speed of compilation and linking (on small machines like Raspberry Pi)
- binary size of anything that might have to be updated or shared over the internet
- having to troubleshooting emergent issues without the ability to lookup documentation or download updates or tools from the internet
In this situation, a simpler language on a simpler VM is probably going to be faster to develop with than compiling/linking a subset of C, and after the initial implementation of the VM, might present less opportunity for an unintended interaction of leaky abstractions in your libuxn and your toolchain to ruin your day on a day when you don't have internet connectivity to check Stackoverflow or to update some buggy dependency.