Iowa speed made the battle line usable

Why did the United States move from a slow battle line to the very fast Iowa class?

The fast ships helped the fleet find, protect, and act on contact with the enemy; a powerful battle line was useless if it could not be directed to the fight.

The slow battle line is the striking end of a sledgehammer, while the faster scouting elements are the shaft that lets it be aimed. A powerful force is of limited use if it cannot locate the enemy or reach the point of contact. Lexington-class carriers extend reconnaissance but may themselves encounter hostile heavy ships in poor visibility or bad weather. The fast Iowas protect those scouting carriers and help translate information into concentrated fleet action at the right place.