The most simple reason is that the BM manoeuvres can be used to boost your Damage when you hit, make a miss into a hit, add special effects on a hit, be used to take special actions in exchange for foregoing an attack... It's more versatile, and while each one is limited to affecting one attack, it also recharges on a short rest, so at the lower levels, it's just straight better.
It's possible that the fact that it affects all of your attacks on that turn might bring it up at higher levels (especially once Rapid Strike comes into play), but the BM still has the advantage of the bonus effects granted by the dice.