With explosives, distance is an enemy. The closer the explosive is to the target, the greater the absorbtion of the explosive force by the target. Creating distance allows the energy to dissipate as it reaches the target. A glacis, as presented by a sloping sandy beach, also helps to channel that force upwards. Butler didn't know this stuff. He also didn't know that the Louisiana should have reinforced on its seaward side so that the energy would be directed towards the land.
The best thing about the expedition to Fort Fisher was that its failure allowed Grant to relieve Butler from his command; thereby eliminating any possibility that the senior most and arguably the most incompetent major-general in the army would succeed him.