I conclude that rather than being a socio-cultural thing, it seems like it would be a pedagogical thing.

Let us consider that fencing consists of 2 broad things:

Firstly, it consists of 5 conceptual principles (namely: cuts/thrusts, guards/parries, timing, distance, and the principles of blade actions).

Secondly, it consists of the particular biomechanics necessary to apply these conceptual principles to a given weapon.

Generally, most systems of instruction will begin with a biomechanically easy weapon that can be used to instruct in all of the conceptual principles of fencing, but then move on to a more biomechanically challenging weapon if it teaches a second. E.g. going from longsword to single-handed sword, since it is easier to propel the weight of a sword with the whole body rather than just the small muscles of the wrist. Or in the 18th c, going from smallsword to the broad sword, since all things but cuts can be learned on the lightweight smallsword, while the broadsword takes a stronger wrist to effectively use.

Now then, a small or mid sized shield does not present a new conceptual principle. it's general action is to permit closing the line on 2 angles in a single tempo, or to close the line and permit the sword to be free to act. This is just a variation of the principles on guards/parries and timing. So a fencer who already has an understanding of these things, already understands what a shield conceptually does, and with creativity, he can understand how to apply this to make his fight easier.

secondly, assuming one already has a biomechanical knowledge of single handed fencing, the shield does not present a fundamentally different biomechanical system. Let a competent single handed fencer take up a buckler, and he'll naturally position himself either in silver's open fight, high seconde with the buckler likewise extended, or in a dardi style guardia di alta.

Strap a targe or rotella to his arm, and again, his natural tendency is to assume the stance proper to these and is seen in all of the texts, as this is the most effortless manner of holding such a shield while also defending himself.

knowing where cuts can come from, and what a most natural manner for holding the shield is, he can quite naturally deduce how to use the shield to it's purpose of closing the line from various cuts.

So there is nothing explicit to really teach about the buckler or any small shield, assuming your pedagogy has already taught the basic principles of fencing generally and a biomechanical understanding of using a sword in one hand. And so, in liechtenauer and fiore, there is no instruction explicit to the shield, and when people in liechtenauer's tradition do teach on the buckler, it's minimal -- maybe a paragraph of plays as examples of fun things you can accomplish. Same is true in broadsword: fencing is taught generally with the small and broadsword, with maybe a paragraph on the targe, since the targe doesn't fundamentally change how fencing works; neither conceptually nor mechanically.

Conversely, when we do see shield-heavy texts, it seems that they tend to teach all of fencing generally through the shield based system. E.g. Manciolino. Manciolino's opera nova only teaches double-weapon fighting, and so fencing's general principles are instilled from the perspective of S&B, rather than progressing through several other weapons as a pedagogical method.

at least that's my opinion on the matter...