well, there are a lot of reasons. tobi is my favourite character btw
a. the timelines with nagato and the bloody mist don't make a lot of sense. kishi had a golden opportunity to resolve the mysteries surrounding the bloody mist, the uzumaki clan massacre etc and with obito he can't.
b. obito got way too strong too fast, he went from no sharingan to ms in one day and complete fodder to top 5 in a year.
c. tobi at every turn gave the impression of someone with a wealth of experience, knowledge and wisdom. his character was incredible and his every action carried a lot of weight, his brilliance and manipulation rivalled orochimaru's, then we get to know he's just parroting everything madara taught him, a big letdown.
d. obito's drive and motivation is based off a small reason. i know he's not doing it just for rin, but come on izuna or kagami as uchiha madara's son, or even madara himself, or the elder son, would have much greater motives.
in short, imo Tobi had the potential to be an amazing character in his own right, and his identity of obito just isn't that. I'd be fine with Tobi having Obito's eye, but the character should have been someone older whose experiences drove him to bitterness, my favourite aspect of Obito (masked Tobi) turned out to be all an act and that's disappointing. Obito's character when unmasked was different.