According to what I can find online, the airspeeder's co-pilot operates the rear tow cable which is used for hauling cargo around and not intended for combat use. The Rebellion resorted to fighting with snowspeeders during the Battle of Hoth because most of the Rebellion's stronger and more valuable fighters were not yet outfitted for subzero combat, with all functioning fighters evacuated as a priority.
Seeing as how Rogue Squadron attempted to shoot down the AT-ATs first with their blaster cannons, it seems the standard approach to fighting AT-ATs was to shoot it down with a big enough gun. They seemed to not know that the speeders' blasters did not stand a chance, so I would guess the Rebellion had not tried fighting the AT-ATs with speeders before.
If the above is true, that probably means the Rebels had not already established the use of tow cables to take down AT-ATs as a known or standard attack method, and Luke was the first to come up with it, probably thanks to his Force-assisted insights.
In that case, why did the Rebels bring a co-pilot whose usual job was to carry cargo into combat?