Originally, way, way back in the day, there was good reason to be so terse. The first UNIX machines had literal teletypes for terminals; printers with moving parts. Every extra character meant wear and tear, paper, ink, and especially, time.
One presumes calling it "src" is in historical alignment with UNIX using short names wherever possible to reduce typing (and the possibility for typos)
See also: "ls" (list files), "mv" (move), "cp" (copy), "/usr" (userland-specific items), "/bin" (binaries), and on and on
Not _the_ reason, but a reason for not changing.
Minor sarcasm above.