Why isn't 1 prime number?

Answer 1

It only has one factor. Prime numbers have two. Also, if one were prime, it would make the fundamental theorem of arithmetic invalid, which I'm pretty sure is a bad thing. That theorem states that every integer greater than 1 either is prime itself or is the product of prime numbers and that this product is unique, up to the order of the factors.

The prime factorization of 6 is 2 x 3. If it could be 1 x 2 x 3 or 1 x 1 x 2 x 3, the fabric of the universe would tear. It's better that one isn't prime.