Is there any rabbinic commentary that explains why God commanded Moshe to write all the words of the Torah on stones instead of on animal parchment?
Devorim 27 (8). Moshe is commanded:
“You shall write upon the stones all the words of this Torah, very clearly.”
This view is not sourced from Rabbinic commentary, as far as I know, however one possible reason why G-d would have Moshe write the commandments on stone as opposed to parchment is that stone lasts forever, whereas parchment eventually wears out or decays. Consider the condition of the Dead Sea Scrolls.
With the commandments written on stone, this ensures that we will always have the stone tablets with the writing of G-d on them. And, when we find the Ark eventually, those tablets will still be in there, safe and sound, with no decay.
As noted by Sam last year, the Abarbanel says that the stones serve as a form of Mezuzah placed at the "entrance" to Israel, but parchment would not.
