Like I said, it's a long shot. But, at the time Obito "died", Madara would have been over 100 years old. A crushed body can be fixed, but a 100 year old body is old.Im going to say the same thing i always do about obito is madara, obitos body was crushed to bits, why would madara want that body?
Obito was introduced to show where kakashi got the sharringan
Also, remember that Orochimaru needed to transfer bodies on a limited schedule. Maybe Madara didn't want Obito's crushed body but couldn't find anything better on short notice. Maybe Madara can only inhabit an Uchiha body.
Itachi did say that Madara was only a shell of his former self with little power. Maybe this is because he is inhabiting a crushed, broken body. Maybe
he had to spend too much power to heal it.
There are plenty of problems with this theory, but it does explain why Madara only seems to have one eye, why he wears a mask to conceal his face (because his face would be recognizable as somebody else), how kakashi got Mangekyo and Kamui, how Madara is able to be alive 100 years after the battle of the end where he was so injured that people thought he died, etc.