Lucy was too powerful.
As an assassin, especially one who approached targets through seduction, Lucy's abilities were simply wasted, a blatant squandering of her potential.
Assassins needed to approach quietly, and leave just as silently, in a job where exposure meant failure. An assassin like Lucy, who went in guns blazing, simply did not exist. Even on the rare occasions when someone like that appeared, they were of little use.
Her remarkable gun skills, combined with a fearless attitude, allowed her to control the situation as soon as she took her position at the door, first dispatching the two most conspicuous men with a couple of shots, followed by another two who dared to move, and then one more bullet to blow the face off anyone who subconsciously tried to aim and fire back.