Because in integral arithmetics, division by zero makes no sense. How do you split 5 into 0 whole parts?
In floating-point arithmetics, division by zero makes sense. It's the scientific way, saying something divided by zero tends to be infinite.
lim x -> 0 (N / x) = ∞,
i.e., if N is divided by smaller and smaller numbers (x), then the result is getting bigger and bigger. It tends toward infinity.