I don't know the actual reason. However, I suspect its for the same reason why they used "bin" for binary, "usr" for user, "var" for varying, and so forth. There was a culture, back then, of not wasting characters because to do so resulted in a real-life waste of space that could limit what was possible (due to storage size constraints). This extended to the fact that the terminal you typed on was sending characters over a 150 or 300 baud modem. Every chance to save some space while still providing reasonably clear communication was considered desirable.