In Arabic, a noun can have three different inflections depending on its role in a given sentence. For example, for the word "book", it can be kitabun, or kitaban, or kitabin. The default inflection of a noun is the "un" ending. (I would guess this would be the same in all inflectional languages—that there is a default state.)

I was listening to a lecturer who enjoyed touching on the nuances not found in an ordinary grammar book. He asked, why it is agreed / decided / happened to be (fill in the word if there is an appropriate term) that the default state of an Arabic noun should have the "un" sound, and not have the "an" or "in" sound?

A question I never thought of.

I would guess, because, maybe, it is the easiest on the tongue?

The lecturer said (to my best understanding), the reason is because among the three sounds "un" sound is the one with the heaviest stress, and one would prefer to have it for the default state.

I wasn't fully satisfied, so I wanted to ask here. Do the experts have a definite or agreed answer to this matter?

(Please help with tagging the question.)