The prime reason for the bulb is the reduction of costs. Fuel is a significant cost for a vessel.
You have to consider that the sole purpose for the vessel to exist is to make money for the owner. If the vessel goes faster, it makes more money Each shipload of cargo earns money. If more shiploads can be transported in a year, the owner makes more money.
But increased speed takes more fuel. That means more cost and less profit. A bulbous bow helps to reduce the fuel consumption (if some other factor had the same or better effect, that would be used, no doubt). That increases the profits. As most of a ship's travels will be on the open seas, the chracteristics at low speed are far less important to the ship's owner.
