Because nothing with a tiny sensor is going to look good in low light. Also the noise at base ISo still precludes a decent print. Even at smaller sizes.
I wish we could shoot raw on this thig!
It obviously still produces some noise (can't defeat physics), but side-by-side with a 4 in the same conditions, the 4S is less noisy. Guess it's using the faster lens to get away with less ISO. Or the sensor is simply built to a higher standard.