Here's why:
* Effect wording: Darkblaze Dragon's effect states: "Once per duel, when this card is Special Summoned: Double this card's ATK." This clearly limits the effect to a single use per duel.
* Card text is binding: The text on a card is the absolute rule for how that card works. It doesn't matter how many times you Special Summon Darkblaze Dragon, the effect only works once per duel.
Even if you Special Summon Darkblaze Dragon twice in the same turn, its ATK will only be doubled once.