One reason: really, really low resource usage. Crates.io is reasonably complex, linking to libgit and such, and still uses roughly 25MB of RAM, constant. A Ruby process is ~30 MB off the bat last I checked, and Rails, well...
Haven't made any direct comparisons (and that would be hard) but from my experience from working with both I would say that you probably won't notice a big difference in RAM usage (< 10% difference).
