The ideological divide is not really between liberalism and conservatism per se. It’s actually between the idealism of the WEIRD Platonic cognitive style vs the empiricism of the holistic Aristotelian cognitive style. (Discussed in more detail here and here)
Moral Foundations Theory (MFT) as elaborated in The Righteous Mind: Why Good People Are Divided by Politics and Religion describes left and right but it does not explain how and why humans, 1) apparently unlike any other species, split into two distinct psychologies, 2) split into those two particular psychologies, and 3) split into the specific relative proportions (about 15% and 85%) that we see.
The roadblock preventing us from answering those questions is similar to the roadblock that for so long prevented a deeper understanding of the “flaws” of reason. Our initial assumption is wrong. Fix the assumption, understanding follows. (The Argumentative Theory).
The incorrect assumption is that cognitive style (e.g., WEIRD vs. Holistic), or ideological vision (unconstrained vs constrained) follows from moral matrix (e.g., one-foundation vs all foundation), as The Righteous Mind implies. This assumption puts the cart before the horse.
Correcting the assumption by putting horse and cart in the right order clears up our understanding similar to the way the right assumption cleared up our understanding of reason. Everything else falls into place. The questions begged by MFT are answered.
The adaptive pressures that cause the split are explained by the natural progression civilizations follow that you described with the Welzel-Inglehart Cultural Map, here. Culture and the psyche make each other up. Holistic Aristotelian empiricism is the evolutionary baseline from which WEIRD Platonic idealism branches off as civilizations move into the latter stages of the progression. (Also discussed here). This baseline -> branching process ALSO explains the relative proportions of Holistic and WEIRD cognition.
But to me it seems more logical that moral matrices are merely markers for, or consequences of, the deeper, more fundamental, root cause of cognitive style. The one foundation matrix naturally follows from the reason-based rationalism of WEIRD idealism, which in turn naturally follows from the latter stages of the Welzel-Inglehart progression.