Why can't I solve this alternate FizzBuzz solution this way?

Given x, if it is divisible by 3, print fizz; if it is divisible by 6, print buzz; if it is divisible by 3 and 6, print fizzbuzz.

Why can't I use 15 like I do in the original problem where I have 3 and 5?

if ( x mod 18 == 0 )
{
    print fizzbuzz
} else if (x mod 3 == 0) 
{ 
    print fizz
} 
else if (x mod 6 == 0) 
{   
    print buzz
}