That was because only 30% of itachi's chakra was given to the sacrifice... which is why he could not use the MS... And samehada was not able to used properly for the same reason too I think...
http://www.mangakong.com/manga/Naruto/260/20
If 100% of obito's chakra was given to a zetsu sacrifice (zetsu hashirama half body power up do not forget)... Along with madara's as well of just him controlling the body... It could create the enemy we saw 16 years ago against minato IMO...