Why are the "ruling elite families" called Reptiles ?

Perhaps, because the old bloodlines, who are also known as the 13 families, have strictly adhered to breeding only amongst themselves over the centuries,

their DNA and genetics have failed to evolve and develop,

they are locked into an evolutionary state that existed thousands of years ago.

meanwhile the rest of Humankind has bred and intermingled with a result of superior genetics, built by nature itself.

It may also explain why they are commonly known as reptiles.

All humans have an area of the brain known as the reptilian brain

In some humans, especially the ancient intact bloodlines, this area of the brain did not evolve as the majority of human brains did, because they always breed with their own bloodlines and one result is that they have over developed reptilian brain areas.

It's only a theory, based in part on the concept of the Triune Brain Region.

https://en.wikipedia.org/wiki/Triune_brain

The reptilian complex (also known as the "R-complex", "reptilian brain" or "lizard brain") was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the idea that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean proposed that the reptilian complex was responsible for

species-typical instinctual behaviours involved in aggression, dominance, territoriality, and ritual displays.

which may also explain their inhuman behaviour.