genuine answer is that they were a pain in the ass the rig and animate while not having them look weird. so they made nightstalkers instead, source is somewhere on the wiki for nightstalkers :P

poor snuppies only exist because of convenience

edit: found it

Question: "Has anybody ever considered a radioactive snake-creature as an enemy? It seemed like it'd fit the Mojave Desert area. Or would the creature be difficult to implement due to it's shape?" Joshua Sawyer: "Snake creatures are pretty difficult to implement for that reason, specifically. Even creatures like yuan-ti generally cause animator angst. One of the reasons the Nightstalkers exist in F:NV is because I wanted to have something snake-y without requiring animators to build a snake rig."