Here's why:
* Effect Text: The effect of Snipe Hunter states: "If this card is Normal Summoned: You can target 1 face-up monster your opponent controls; banish that target until the End Phase."
* Normal Summon Restriction: This effect triggers only when Snipe Hunter is Normal Summoned. You cannot Normal Summon more than once per turn.
* "Once Per Turn" Restriction: The effect doesn't explicitly say "once per turn", but the wording "If this card is Normal Summoned" implies a single use per turn.
Therefore, even if you have multiple Snipe Hunters on the field, you can only activate its effect once per turn.