Snakes would indeed be awesome.
Unfortunately they're impossible in the engine for the same reason that all the elementals in Oblivion had to be humanoid rather than big amorphous bonfires: The engine only supports legged motion.
Unlike some other games it can support any regular number of legs, 2, 4, and 6 are all represented in-game, even one-legged motion is supported (securitrons) but no-legged slithering would cause a real issue for the engine I assume.
you forget there are hovering enemies. They probably experimented with snakes that hover an unnoticible height above ground but whenever they went over uneven terrain it looked terrible or they kept getting stuck in rocks and things so they hit the 'make extinct' button and went back to the Mariposa Laboratory to create a substitute:
Rattledogs!
so the mutated rattlers had to grow legs to survive in a world where only legged motion is possible. makes perfect sense to me. Hence the mighty Ratttledogs...that lay eggs...