Jovius Valerius scripsit:

Originally the sacred triad of the Roman Religion included Jupiter, Juno, and Mars. At some point Mars was replaced by Minerva. Does anyone know when this happened? And, more importantly, why?

How early is 'originally'? According to the entry on 'Capitol' in [Hornblower, p. 288]:

It is best known as the site of the great temple begun by the Tarquins ... and dedicated, in the first year of the republic according to tradition, to Jupiter Optimus Maximus, Juno, and Minerva.

So if there was a Mars to Minerva transition before this, it happened in pre-republican times under the Etruscan monarchs. Whats your source for this transition?

I agree that why is an interesting question, but first we need to pin down when.

Hornblower, Simon; Spawforth, Anthony (eds) The Oxford Classical Dictionary 3rd revised edition, 2003, Oxford University Press.