In general, lower code complexity results in fewer bugs and security problems, less code implies lower resource usage, so you could run the system with a smalle

In general, lower code complexity results in fewer bugs and security problems, less code implies lower resource usage, so you could run the system with a smaller main memory, a slower CPU or lower power/energy consumption. In addition, if you plan to target real-time applications, setting up Linux to guarantee your real-time constraints is not trivial. All of this is not that relevant for running Smalltalk-80 on the Pi, since Smalltalk was running happily on 16-bit machines with one or a few MBs of RAM. So I guess the ultimate reason for doing this is "it can be done" - for the fun of hacking :-).