Simple answer here... because with some babies if you don't introduce them before 12months, there is NO way they will try to drink anything BUT FROM A bottle & for me as a bf mother who had bottles for ebm with ds#1 from 6mo, it was a bugger to get him to drink milk from anything other than a bottle, even though I had him drinking water from a sippy.... I have many friends who have had that problem, their lo's will take water, but not milk from a sippy.
with ds#2, I started at 6mo, giving him a small amount of ebm in a sippy, with solids & now at almost 11months he takes 1 sippy of milk a day & still has 3 bf... he also drinks from an open cup & he doesn't "suck" on anything else except his bf & they are 3x5mins.