At one level: because the root word of quadrant means four/one-fourth, so once he got to 4, he did run out of numbers [of quadrants].
He probably picked to use quadrants in large part because categorizing people’s broad behavioral tendencies into a billion groups isn’t that helpful of a mental model simplification. “People in group 24,825,726; those are the tattletales who like Brussel Sprouts, are left-handed, allergic to oak pollen, love dogs, hate cats, taste quinine, can roll their tongue, are AB+, and prefer sleeping in cool rooms.” How does that make for a readable essay?