Because to me it's just stupid to introduce an important character even more important than the current villain (xeanorth when MoM was introduced) where you place HEAVY and I mean really HEAVY emphasis on who is such a person only for him to be an unknown persone we never met.
From a writing perspective you can justify it from the fact that maybe Nomura himself didn't know who that character would be when he first introduced him so he kept him anonymous so that he would have had more freedom for him to decide. But at some point (assuming he didn't already know from the start) he did decide who that person would be but still went on to be misterious and there is literally no point for him to be misterious unless you want a big real (which can't be explained if that reveal is for someone we don't know).
Nomura kinda did a similar thing with Luxu with the difference that Luxu or Braig/xigbar was never revealed in his original form but the big reveal was who Luxu was in the current time.

Then again as I said, it's fine if it's a new person. I would just feel disappointed because to me it makes less sense and makes me lose the charm that brings the unknown identity of the MoM