Actually, I've wondered about this myself.
Bethesda implemented diseases into their previous Gamebryo titles (eg. Oblivion), yet they somehow didn't in a gameworld where stuff like cholera and the bubonic plague would run rampant.
Maybe because it'd be too difficult/politically incorrect to have real-world diseases?
Yeah, remember the diseases from Oblivion? They basically had almost no effect. They were a form of resource attrition- if you were wandering around in the wilderness, you had to carry potions of Cure Disease or suffer stat penalties after extended roaming. In NV, that gameplay role is taken by Food/Water/Rads (and Ammo/Stimpaks to a lesser extent, as your attacks and healing no longer rely on regenerating magic).
Plus, the games aren't supposed to be simulations of reality down to the finest detail- they're designed around the themes of the setting. When you think of Fallout, you don't think of getting dysentry, you think radiation and mutation.