Nonduality is, of course, advaita (“not-two”). The end point of the advaita teachings is Self-realization—enlightenment or “liberation”. This is said to present when the indvidual’s identiy is dissolved permanently into the omnipresent and unlimited Oneness (aka Self); realizing the true nature of the self.
When this presents, it is as if it was—to the individual—a miracle. It is thoroughly life changing, and in fact world-changing. So that is, primarily, why Self-realization (or advaita) is taught.
I would summarize your query as “Why teach advaita when you could be teaching ajata?” I do teach ajata to those who seem to understand advaita. And I have also (after thirty years) begun writing about it.