quantum mechanics - Compactification of dimensions in string theory: Why our Universe has 3 large spatial dimensions?

Is the only way that string theory can respect the principles of quantum mechanics, and Einstein's special theory of relativity, is to formulate it in a hypothetical nine dimensional space?

You could use string compactification for $d$ number of non compact directions, all that is required to have $d$ non-compact directions is to choose an appropriate compact space of dimension $9-d$.

The question that arises from that is: Why does our Universe have 3 large spatial dimensions?