My understanding was that because the 8th day is d’rabbanan and Gebrokts is a safek and safek d’rabbanan l’hokil.
Furthermore we are meikil to show that Gebrokts is a veite chashash and that those that do not keep it are not, chas v’shalom, eating chametz.
Chachem, there are many minhagim which are post-Talmud in origin.