Because even if one can work around the momentum-conservation issue, they haven’t been invented yet.
Simple as that.
Wait sorry, I thought a vector control drive was basically a gravitic drive that pulls on the observable universe?
Yeah, for very approximate values of (mass within the) observable universe - but that it pushes/pulls on anything means that it’s just another kind of reaction drive, albeit a nonlocal one in the same way as a tractor-pressor. A reactionless drive would, definitionally, not need anything to shove against.