It is not that folks didn't consider such ideas, but that their proponents were usually heretics who posed an existential threat.

From Philosophical Ideas Behind Their Time by  Alex Tabarrok.  An idea behind its time:

What is less commented on is the third possibility, ideas that could have been discovered much earlier but which were not, ideas behind their time.

His list of philosophical ideas, behind their times.

Hume’s is/ought distinction: the idea that you cannot derive a normative conclusion from factual premises.

Hume’s problem of induction: past regularities do not rationally guarantee future regularities.

Rawls’s Veil of Ignorance: the principles of justice should be derived without knowing one’s own particularities of class, race, gender and so forth. Seems obvious as an idea.

The Trolley Problem: similar ideas can be found earlier but the clean distinction between killing and let die or more generally omission and commission could have come much earlier. One might also think of the Prisoner’s Dilemma in this category of ideas or constructs that cleanly isolate an otherwise present but opaque idea.

The analytic/synthetic truths distinction: some things are true by definition, others are empirical. Obvious and it can be found before say Kant, yet a clear earlier statement would have resolved many issues and seems well within say Aristotle’s capability.

Aumann’s Agreement Theorem, technically, this requires Bayesian machinery and is difficult to formulate with precision, so I would not say the actual theorem was behind its time. But the underlying idea—that disagreement itself, not merely the arguments offered, should cause one to question and refine one’s own beliefs—could have been developed in Athens.

I’d also nominate a package of ideas like abolitionism, equal rights for women, and religious toleration–each of these is tendentious as examples yet the basic package seems fairly obvious as a category and yet late. (Perhaps if the veil of ignorance had been thought of earlier so would these ideas!) 

Note, that I am not arguing that abolitionism or equal rights for women could have happened much earlier only that these ideas were behind their time–the ideas were morally obvious even if not institutionally feasible.

A marginally interesting exercise but with a strong cast of intellectual onanism.  My first thought was that this could devolve into arguments of definition, and there is an element of that.  But it feels like there is more of a critique than that but I couldn't put my finger on it.  Something about the whole framing of the idea.

But commenter Sure gets at the issue.

Abolitionism as an idea dates back to at least the patristic era, if not Zoroaster. Religious toleration was not just an idea but Achaemaenid policy. Equal rights for women was largely opposed by women (it wasn't until WWII that majority of woman actively mobilizing on Woman's Suffrage were women), but it was policy among the Shakers, if not the Beguins.

The distinction between killing and let die dates back to at least Hammurabi. The Talmud explicitly condemns suicide, but allows one to undertake actions that lead to someone else killing you.

The veil of ignorance was proposed and rejected as incompatible with an all knowing god.

Hume, likewise, hcas historical antecedents (e.g. Socrates), but runs into trouble whenever people accept personalized or personalistic god(s).

And that is the big problem for a lot of this. It is not that folks didn't consider such ideas, but that their proponents were usually heretics who worried the establishment that their whole philosophical package would lead to a decay of morals, the dissolution of the people and the waning of its military power, and lead the populace to forsake the proper worship of the divine.

You can find all manner of folks who took positions that spring readily from the idea that any deity is remote and not informing people how to live. But that idea package has a terrible historical record. Rising copies of Stoicist literature, for instance, is associated with some pretty nasty demographic troubles. The Minha gave rise to a bloody rationalist persecution followed by a counter purge.

In a premodern world you need blindingly high birth rates, extremely high willingness to violently oppose interlopers, and a willingness to risk death for the community to stand up to the invasions on the horizon.

The folks who adopted the precursors to these ideas, generally, were crushed meretriciously by those who didn't. And the folks willing to be violent enough to uphold these sorts of concepts against the hordes of true believers? Well since the French Revolution that has typically been bloodier than their contemporaneous religious counterparts.

The real magic sauce here is not that these ideas might come about, it was that there existed societies confident enough and overpowering enough that they could indulge the whole project without either demanding a Socratic suicide or be overrun by some place more willing to do what was needed to sustain greater violence.

The Protestant West was weird. It tolerated heretics of numerous stripes without falling into demographic death spirals, too much internal conflict, or any of the very common failure modes that saw enterprising humanistic philosophers get executed by their compatriots or those beyond the borders.

I am exceedingly doubtful that any of these ideas could ever reach wide sway for centuries in other societies. The Inquisition would root such folks out and, at minimum, ostracize them. The Islamic world tried this and had one of the more famous purges in its history. Even among the Chinese, this sort of setup gave rise to incredibly harsh Legalistic practices.

And as an aside it is blindingly hilarious for Alex to invoke Bayes without at least considering the option that paucity of societies that endured with some of these concepts might bespeak something about their survival utility (at least under ancient conditions).