It is because making the Dealer the small blind maintains the features that create the action whilst counterbalancing the power of position.
Going from 3 or more players to 2 players something different will always happen,
simply because there are not enough players to maintain the 3 specific positions usually seen from the dealer button, SB, BB, UTG;
(on a fuller table there would then be the others up to CO and button).
Notice something odd even occurs at 3 players because the Dealer appears to act first being in the UTG position without any chips invested.
After the flop though the usual process of the player to the left of the dealer acting first resumes.
Notice also at this point that the player with least forced chips invested acts first preflop but then has position, (the opportunity to reflect on the other players actions before deciding on their own move), for the rest of the hand.
So this helps to even out each hand for the players involved.
Interestingly it could even be argued that there is still no actual change when it gets down to 2 players.
We usually assume that the player after the dealer is the small blind and the next player the big blind.
However if we look at it from a different angle, the small blind and Big Blind have to be posted,
the Dealer Button has always been placed in the most favourable position after the Big Blind,
so with just 2 players this would be the small blind, least amount forced,
(obviously necessitating under the gun like first acting), but then having position for the rest of the hand.
So in conclusion when you sit back and start to think about it fully it actually all becomes perfectly rational.
You really have to hand it to those texans for coming up with a fully rounded game,
what a shame they didn't invent tennis, then there might not have been such a to-ing and fro-ing of service games, which is what would happen in heads up poker if one position had more of a significant advantage over the other.
