Rashi on Esther 9:10 said they didn't take the money of the allies of Haman, so as not to arouse the jealousy or envy of Achashverosh, over the new wealth.
In other words, it was a political consideration not to take the war booty.
We also find that David did take benefit from the war booty that he captured when he fought Amalek, so this is a precedent that it is allowed.
But then the question arises, if so why was G-d mad that Shaul{Saul} took part of the sheep from the war booty of Amalek. Is it because the prophet specifically told him not to do it in this situation, or is there some deeper reason which sometimes allows us to take from Amalek war booty and sometimes no.
I have yet to find the answer.