Why isn’t 1 a prime number?

When I write the definition of prime in an article, I try to remove that ambiguity by saying a prime number has exactly two distinct factors, 1 and itself, or that a prime is a whole number greater than 1 that is only divisible by 1 and itself. My mathematical training taught me that the good reason for 1 not being considered prime is the fundamental theorem of arithmetic, which states that every number can be written as a product of primes in exactly one way. In the positive whole numbers, each prime number p has two properties:

The number p cannot be written as the product of two whole numbers, neither of which is a unit.

Source: blogs.scientificamerican.com