Why is it The answer to life is 42 on Wikipedia?

because the hitchhikers guide to the galaxy says so

another answer:

This is an ironic joke from the Hitchhikers Guide to the Galaxy, written by Douglas Adams. When a supposedly all-knowing SuperComputer is asked the meaning of life, the universe, and everything, the answer given is: "42." In another context, Restaurant at the End of the Universe, the ultimate answer is "6 times 9, or 42." Of course, 6 times 9 does NOT equal 42, and therein lies the irony.