Question

probability of an event is always between 0 and 1. why?

Answer 1

because no. of possible outcomes can never be greater than total no. of outcomes
and no. of possible outcomes can never be negative

Answer 2

.$\huge\red{answer\::1\:or\:0}$

The reason is that we use probabilities to multiply against other values. The range between 0 and 1 is special because 0 "of" something is 0 and 1 "of" something is itself (with all other values mapping smoothly between). So by mapping probability functions to the range [0,1] we present them in the most immediately useful form, where they can be multiplied by a quantities to generate related results.