The diapsid infratemporal fenestra is homologous to the synapsid single temporal fenestra, so this asserts that the amniote line first developed a single fenestra (the [infra]temporal fenestra) and then later one lineage gained another (the supratemporal fenestra), with this lineage becoming the diapsids while the other being synapsids.

With this in mind, why are the early amniotes who first developed a fenestra not considered synapsids? Is it to maintain synapsida and diapsida as monophyletic that we only consider them synapsids a while after their defining trait developed?

Or, rather, why are diapsida not considered a clade of synapsida given that the common ancestor of both lines already possessed the infratemporal fenestra? It would seem sensible to me to put them both in a clade specifying a single fenestra ("monapsid"?) with the synapsids just being those more closely related to [insert synapsid here] than to diapsids.