i----------->.
1-Main Reason :
Teh reason we thought Tobi is Madara ...it's becuz of one simple Reason :
-SIMPLY BECUZ HE WAS INDEED UCHIHA MADARA ...
other secondary reasons :
minato, kisame and itachi said he was Madara so yes he is Madara...if kisame and itachi and minato say tobi is...