This is just my opinion, no official statement about it has ever been issued, but, I think it was a problem due to the fact that kids were getting confused and didn't understand why the M&G for Mickey didn't have a Mickey that talked to them. I can understand a kid getting upset by that and wonder why they just saw Mickey talking but now he wouldn't talk to them. The problem of getting people the right size to fit the Mickey position that also could easily operate the mechanics was difficult as well. There was no consistency of quality and the voice was another impossible way to keep consistent. It was a noble effort, but I feel that the problems vastly outweighed the positives. Mickey had been making the rounds for 60 years without talking with no one really complaining about it. They were better off to just ditch it and go back to the way it was.
Remember this is just speculation on my part. I don't know why for sure, but, what I just said makes sense to me anyway.