past participate vs past

In the following paragraph the green sentence is indicating a period of time before the time of the red sentence. So, why the author has not said "which hadn't altered" instead? Because "which didn’t alter" has a more continual sense than "which hadn't altered" and therefore conveys more a period of time than a certain point of time?

Context:

"Mithen’s speculations are based on the scant evidence of the archaeological record but are informed by insights into the workings of the modern mind, and his approach is illuminating. He identifies in the early human brain four distinct kinds of intelligence, which functioned discretely: a natural history intelligence; a technical capacity; a social intelligence; and the ability to acquire language. A natural history intelligence reflected an intimate acquaintance with the landscape – with the weather, the seasons, with plants and animal behaviour. Technical intelligence, which didn’t alter much for millennia, enabled early humans to make tools, forming within their minds as they napped away at a piece of stone, a mental template of what they were
aiming at."