Hashirama Cells possess extremely powerful vitality, making it quite difficult for Madara to fuse with them.
It is no easy feat for the two to be compatible. In the original story, Madara only fused with that tiny bit of physical attributes on his chest, and that was already enough for him.
Orochimaru guessed that it probably wasn't that Madara didn't want to cultivate more of them for fusion; given his strength and the resources at his command, it was not something he couldn't do.
Chapter 256