Dickinson is suggesting that the goal is most attractive to those who strive mightily but fail to attain it; "nectar" will most completely understood by those who have "sorest need" of it but are unable to reach the goal.
Her illustration of this idea refers to warriors after a battle. Those who are on the victorious side - "Not one of all the purple host Who took the flag today" - can't truly appreciate the success they have achieved. The one who completely comprehends the price exacted in gaining the winning side of the battle is the one who lies "defeated, dying." This is the warrior who really knows the meaning of the "distant strains of triumph" but will never be able to share in the sweet success of the battle's outcome.
Nectar is not a forbidden fruit; it is an unreachable desire.