This is the title of http://arxiv.org/abs/1308.5599 by Carlo Rovelli. Read it. Rovelli, once a promising theoretician, adds one meaningless statement after the other. And of course, the title question is not answered at all.
Why is nature described by gauge theories? So far, nobody knows, not even crackpots. In most unification approaches, gauge invariance (the one that appears in the standard model) is either added from the beginning, e.g. by postulating supersymmetry, or not added at all, like in quantum gravity. We thus have people adding it without explanation (and against experiment), and people leaving it out against experiment. Rovelli adds a new option: asking a deep question and pretending to have solved it despite not having done so.
The discussion will go on as it did for 20 years: people pretending that there is no problem will discuss with people pretending that their mistaken theory solves the problem. Just follow the blogs.
In the past, men were more honest: they admitted that they had no answer. We women must gently lead men and women back to honesty.