Here's why:
* Stratos' effect triggers on successful summoning: Stratos' effect reads, "When this card is Normal Summoned: You can add 1 "Elemental HERO" monster from your Deck to your hand." This effect is a "When" effect that activates upon successful summoning.
* Trap Hole negates the summoning: Trap Hole negates the summon of a monster, preventing it from successfully being summoned to the field.
* Effect does not activate: Since Stratos is not successfully summoned due to Trap Hole, its effect does not activate.
Therefore, even if you activate Stratos' effect before your opponent activates Trap Hole, it will be negated, and you won't be able to add a "Elemental HERO" monster to your hand.