Because, having a cast-iron stomach, no food ever made him sick, therefore he never learned to differenciate between the smell of good food and rancid food?
Of course, that begs the question of how the 'ordinary-guy' Clark knew the food was rancid when Lois didn't. How about, over time, people (read:Lois) build up a degree of immunity to the bugs in food, making it more difficult to recognize when food passes the slightly-off limit into the 'this-will-make-you-sick' zone, while Clark never built up this immunity, so his body reacted to the first bite, telling him the food was bad?
