Why does irrational numbers don't stop?

Because if they stopped they could be expressed as a ratio.

Suppose the decimal expansion of an irrational stopped after x digit AFTER the decimal point.

Now consider the number n, which is the original number, left and right of the decimal, but without the decimal point. This is the nummerator of your ratio. The denominator is 1 followed by x zeros. It is easy to show that this ratio repesents the decimal expansion of the number