MJ struggled with his voice on tour all the way back to J5 era. It's a big strain on the vocal chords singing 2 hrs live every night. As you get older it gets worse. Ad not sleeping on top of it and you are going to damage your voice. The Bad Tour took a toll on his vocal chords.

That's why he lip synced more songs on DT, but he didn't sleep and the allegations made it worse. In 93 you could hear the difference in his vocals. This is one of the reasons why he didn't do more than 3 solo tours and skipped 80% live vocals on HT.