Be cause she didn't want that baby drinking....cow's milk.
Okay....here's really why. The suckling kitten stimulated her into producing milk. It's that simple.
A dog we had...thought a kitten we'd gotten was her baby. She carried Alex everywhere. Let him nurse...and even went so far as to "clean" up his poop (eat it...) I told her...she didn't need to be that good a mother.