Why are humans so amazingly bad at reasoning in some contexts, and so amazingly good in others?

From the introduction to an Edge 342 discussion with Hugo Mercier, one opf the authors of the "argumentative theory" of reasoning.

'Reasoning was not designed to pursue the truth. Reasoning was designed by evolution to help us win arguments. That's why they call it The Argumentative Theory of Reasoning.' So, as they put it, 'The evidence reviewed here shows not only that reasoning falls quite short of reliably delivering rational beliefs and rational decisions. It may even be, in a variety of cases, detrimental to rationality. Reasoning can lead to poor outcomes, not because humans are bad at it, but because they systematically strive for arguments that justify their beliefs or their actions. This explains the confirmation bias, motivated reasoning, and reason-based choice, among other things.'