Complex sentence: It is because: there are 5 degrees of freedom in quantum mechanical mono waves, if one is not pre-occupied with Minkowski spacetime, then the spacetime which accommodates all mono waves most elegantly would be 5d.
Split into simpler sentences: It is because: there are 5 degrees of freedom in quantum mechanical mono waves. If one is not pre-occupied with Minkowski spacetime, then the spacetime which accommodates all mono waves most elegantly would be 5d.