I don't know if you've been to the D-Day beaches, but if you see them, you'll see that there's nowhere really to hide. Unlike British coastal defences, the German plan for the Atlantikwall was to stop the Allies on the beaches, or if possible to prevent them landing at all. Rommel believed that if the Allies had any foothold on the beaches after 24 hours then the battle would have failed. The defences on Normandy are therefore designed to keep an enemy confined to killzones on the beach to be smashed by inland artillery. With overlapping fields of machinegun fire in particular, using smoke would have been entirely ineffective, as all that would happen would be a cloud of smoke still entirely contained within a killzone, and still hit by artillery.

The Allied plan, of course, adapted for this by planning for airborne assaults and overwhelming air and naval superiority to remove the threat of artillery and bunkers as much as possible, as well as specialised armoured vehicles to punch a hole in the beach defences and get the infantry and tanks off the beaches and out of the killzone as quickly as possible.

Assault Plan, United States Army Center of Military History (online).

Ambrose, S. (1994), D-Day, June 6, 1944: The Climactic Battle Of World War II