Because BC stands for "Before Christ." This dating system wasn't used until after Christ had been born.
Try these:
A baseball team scored four runs in one inning, but not one man reached home. Why not?
A round house was robbed. The son says he was playing a video game. The maid said she was cleaning the corners. the mom said she was cooking, and the daughter said that she was in her room. Who was the robber?
If a bungalow is red, and everything in it is red, what color are the stairs?
What do you call something with a head, a tail, but no body?
See more BRAINTEASERS AND MIND STUMPERS with answers at Not Just Nuthin.
Check out 10 Brainteasers To Test Your Mental Sharpness featured at Forbes.