Lack of precision-guided ordnance is the first answer. Yes, the Navy of Britain and the USA (and the surviving French as well) definitely shelled the heck out of those beaches at Normandy prior to the troops "landing" and getting a foothold there, but this was 1944 remember. The unguided artillery fire from the ships offshore still had a wide CEP (Circular Error of Probability) factor that meant that many of the bunkers and pillboxes were left untouched, or at least left many of those defensive fortifications at/near the beaches not directly struck and destroyed by incoming munitions from those ships. The ships offshore cracking off that support fire had a lot of trouble (due to weather conditions and tide conditions that could have "grounded" them), not wanting to risk getting too close to the beaches, and that meant that the assault ships (destroyers and battleships and light cruisers) were in a lot of cases just "firing blind" and blasting away, with their ranging figured but not the accurate targeting capability to strike enemy emplacements.

What this "CEP" and lack of targeting factor, affected by tides and shoal effects on the beachhead, meant was that the offshore ships could not get close enough to precisely target and engage those bunkers/pilboxes in the Normandy area and strike them with decent accuracy with the weapons of the day (I believe the big ships delivering that artillery had to stay 2-to-5 miles offshore as per orders to avoid being grounded), and instead they had to try to "pepper" the assault beaches with many multiple rounds prior to the first wave of allied manned invasion forces landing. This was just an example of the technology of the time, which, while used to somewhat decent effect, still failed to take out many of the defensive German positions on the various beaches being assaulted on that fateful day. The aircraft used as air support for the D-Day assault also could not hit the ground targets with much degree of accuracy (due to the fact they were dropping unguided munitions, whose aim was hampered further by the crappy weather and low cloud cover on the day of the invasion, which negatively affected air support efforts). Also, the allied "mapping" data to pinpoint the precise location of German counter-batteries was not really complete or totally accurate pre-assault, so that also added to the insufficient naval gunnery support conditions on D-Day.

In contrast, the Iwo Jima invasion was conducted in good visibility conditions, and the US Navy had conducted massive air raids and naval artillery attacks against the island in the days prior to the manned invasion itself, because the element of surprise was not deemed a factor in that invasion. During D-Day in Normandy, the troops assaulting the beaches there were afforded no such luxury, a concession designed by the planners in order to retain the element of surprise. The element of surprise was deemed to be a vital component of the June 6th, 1944 D-Day assault on the Normandy beaches, and that was not the case in Iwo Jima.