cause teach is still strong. also, i wouldn't take oda's words literally. already at sabaody, he told us roger and crew discovered the hidden history at laugh tale. maybe we don't get to see the one piece, but they need to reach laugh tale to understand what happened to joyboy. and then draw their conclusions, like rayleigh said (which i imagine means, they chose to wait for someone else, luffy will choose to fight).
he could do another roger's flashback and skip the reaction to the one piece, and show what it was in the last page.