Why Did It Take So Long to Discover Zero?

I believe that the answer is simple. Zero is a very unnatural thing.

I think that the only number naturally attributable to physical objects is one (1). The number zero (0) indicates nothingness, and there exists no "no physical object": Physical objects can only naturally exhibit their existence (“I am a brick”). They cannot exhibit their non-existence (“I am a no-brick”). You can show me one brick but you can't show me zero bricks (if I can't see it, you haven't showed it to me).

The number two (2) involves an abstract coupling of two physical objects (one object and one object), but abstractions are ontologically subjective, that is, abstractions don't exist without intelligent beings (yet the number two is pretty much an epistemologically objective thing--that is, there's not much disagreement about the number two and what it means).