Texas Hold'Em Heads-Up Blind Structure
Basically, as Wikipedia hints at, to make sure that a different player acts first before that flop than after the flop — just as in non-heads-up, where “Under The Gun” (the player after the Big Blind) acts first before the flop, and the Small Blind acts first after the flop.

In heads up, if the non-dealer would post the Small Blind and the dealer the Big Blind, then the non-dealer would act first both before and after the flop.

I guess it would be possible to “reverse” the rule, and instead “simply” let the dealer act first after the flop, but that would be a rule exception after the flop, so if someone would make a mistake in the order of play it would probably be more complicated to “clean up” or “reverse” that mistake after the flop than before it.