The rumors in London were that LMK wanted to leave the UK production and have some time off prior to taking the show to New York, but the producers wanted her to stay right up until rehearsels for Broadway began. She wasn't willing to do that, left the show there, and that's history.
It could have been (and most likely was) an Equity thing. I doubt we'll ever know for sure..
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.