To my knowledge, he never said why. Some people suggested that it may be because Richie had said that MFOJ was his favorite of the songs Jon wrote without him. For some reason, I think it has more to do with what Jon said in the boxset liner notes about there being some songs that just made you too sad when you think about why you wrote them (paraphrased from memory).

Regardless of why he said it, if Jon wasn't lying and really has no intention of singing it without Richie, then it still tops my list of songs I'd like to see the band do in EU... (or Israel, or on a cruise, or on a train, or in the rain... I'll stop now; since Dr. Seuss has apparently taken over my brain.)