That is just the way mathemations defined it. Because for zero the factorial is undefined, the reason why mathemations defined it that way is because a lot of time in, for example, infinite series the denominators the the following pattern. 1,1,2,6,24,120,.... You see the pattern? Thus, if we define 0!=1 then the pattern is easily as 0!,1!,2!,3!,.....

Also there is a way to further define the factorial for non-negative number, that is called the Gamma function. And in the Gamma function, the function of 1 is 0!=1.