When Dr. Brigid Tenenbaum & the other scientists were trying to find a way to make ADAM faster and in larger quantities, they discovered they could use Little Sisters by trial & error. They tried making Little Brothers, but they turned out being very violent and aggressive, & therefore unfit for the job of harvesting.
As far as being cut from development, the makers of the game wanted the harvesters to be something that you as the player could relate to & empathise with, because they wanted the choice of harvest vs. rescue to be difficult for the player.
If you look at very early concept art, the original plan was for the harvesters to be small furry chipmunk like creatures. But then they got the idea to use little girls. And of course, little girls would be quite a bit easier to empathise with and pity.