this questions is for fans to guess on this topic why they cut these things:
http://tcrf.net/Fallout:_New_Vegas/Unused_Creatures#Wind_Brahmin_.2816938d.2C_17654f.29

check out that page its werid but funny RadioActive Tumbleweed lolol

but its for developers if they get a chance why they cut some of those npcs or quest or etc?

and its for fans to guess

Not enough time to put it in the game. They came up with the idea and then desided against it. Maybe Bethesda said no. To much content and had to make some cuts to save space.

Those are some likely reasons that come to mind.