Many reasons, among them:
- Tides and beach obstacles - in Normandy, high tides comes at night. The Germans had expected night landings as a serious possibility and so had seeded the beaches with mines and obstacles (see this diagram) that would snag or blow up landing craft at high tide. By landing in daylight, Allied landing craft could come ashore and lot tide and avoid most of these obstacles.
- Confusion in darkness - the British and Canadians had already attempted a night (technically a very early morning) landing during the Dieppe raid of August 1942. The attack had been plagued by confusion only exacerbated by the bad light - several units had landed in the wrong place, etc. Landing in daylight made it more likely landing craft would find the right place (and even that wasn't guaranteed the entirety of 4th Infantry Division landed on the wrong "Utah Beach" when they drifted off course.
- Naval bombardment - the naval barrage began just before daylight using dead reckoning - but when dawn broke, gunners could see and hit their targets much more accurately. During the landings, several destroyers also got within a few thousand yards of the beaches to deliver gunfire support to the battered troops on the beaches. Although this was improvised and not a planned part of the gunfire support plan, this kind of up-close support by the DDs would have been impossible at night.
This is not to say there were not considerable night operations prior to the morning landings.
- Three airborne divisions (the Americans 82nd and 101st and the British 6th) landed in Normandy during the night to seize bridges and beach exits.
- Frogmen and demolition teams swam ashore to begin demolishing and marking beach obstacles.
- Men of the COPP (Combined Operations Pilotage Parties) helped lead the British I Corps to Sword and Juno Beaches by waiting offshore with the support of X-craft midget subs and deploying radio beacons and marker lights. The COPPists had also been ashore several times in the months previously to survey the beaches ans assess their suitability for landing.
- Minesweepers cleared German naval mines beginning around midnight and didn't finish until just before dawn.
- Immediately prior to the landings at night and in the early morning, Allied bombers and fighter-bombers attacked German defenses near the beachheads and coastal artillery batteries. They also hit radar stations - 74 out of 92 targeted German radar stations were knocked out, hampering the German's ability to determine if the Normandy landings were "real" or just a feint. All in all 360 heavy bombers from the 8th Air Force and 269 medium bombers from the 9th Air Force took part in the raids, although heavy cloud cover ended up limiting their effectiveness.
