It seems that a lot of lives could have been saved if a aerial campaign would have been launched before we sent in ground troops. Why was this a bad strategy?
They did: it was called the Transport Plan, and it was absolute hell for SHAEF to convince the RAF and USAAF to commit to it. In the week prior to the landings the Allied air forces committed to bombing railway hubs, roads, and bridges (especially over the Seine and the Loire) to seal off Normandy and delay the arrival of German reinforcements when it became clear that this was the invasion. It worked extremely well, delaying the arrival of the reserve panzer divisions that might have severely limited the beachhead the Allies could expand. It also nearly didn't happen: the RAF and USAAF saw their niche as being strategic bombing, which they were unjustifiably convinced could win the war on its own. They were very young services (indeed, the U.S. Air Force would not become an independent service until 1945) and despised the threat to their independence implied by being accessories to ground troops by conducting close air support, interdiction, and transport.
Sources:
Max Hastings, Overlord
Charles Messenger, The D-Day Atlas