Once a Pacific island was surrounded, its garrison was effectively under siege and could not replace losses or withdraw beyond the objective. Normandy remained connected to a continental army able to retreat, disperse, and reinforce. Extending the bombardment would therefore sacrifice surprise and allow mobile German forces to avoid the fire rather than wait on the beaches.
Notable Point · 3:00:53-3:01:23
Why Normandy could not use a Pacific-style bombardment
A multi-day naval preparation suited isolated Pacific islands but would let German forces move away and return to reinforce Normandy.