According to the Oxford English Dictionary (OED), in the 19th century hardcore referred to rugged material suitable for use in applications such as the foundations of roads. By the early 20th century, it had taken on a metaphorical meaning, used as an adjective or a noun to refer to the most unwavering or intractable elements of a group, and in particular to the most diehard or stalwart adherents of a movement or ideology. By the late 1950s, it was also being used to denote extremism of another kind: explicit pornography.
Terms for philosophical or aesthetic radicalism tend to build on spatial metaphors that emphasize distance from the center or mainstream: fringe, edgy, extreme, far right or left. The word hardcore is an exception. It envisions radicalism as the center: the nucleus of a peripheral sphere, rather than the periphery of the norm.
From there to here—"When the noun core is used as a modifier, as in 'core values' or 'core beliefs' it designates essential and universally accepted qualities—the antithesis of the radical fringe"—and then to normcore, in which the "-core" suffix (in my view) takes on the polar opposite (and thus, sort of the same) signification as in hardcore: anti-radicalism at the center, and whatever radicalism that entails. [OD]