What was the reason that Luke found for paying attention on fashion, that needed an insight into the reasons people care about fashion?

I would imagine it would depend on the image you're trying to present. Like Luke said, it conveys "large packets of information about [him] at light speed." So, if you're trying to show you have money you would dress one way, trying to look cool would require a different style of dressing, and just hanging casually with friends requires a third.

There is a tendency for many rationalists to try and look for a reason for peoples behaviour and, when failing to find it, to try and find an evo psych explanation for this behaviour.

Yes, I agree. I mostly use evo psych explanations to increase my internal probability of a PUA trick actually working, though I'm conservative about the credence I give to any evo psych explanation.