My bet is that Mearsheimer and Walt were paid off by someone
Alan Dershowitz has written a devastating response to Mearsheimer and Walt's paper on the Israel lobby. As they would say in middle England, "God bless his soul."
At the end of his working paper, Dershowitz asks the poigniant question why? Why, given all the obvious inaccuracies and false evidence recycled from discredited sources, would these two academics publish such garbage?
My suspicion (with no hard evidence) is that was someone who hates Israel/Jews who was willing to pay them to do it. That's why Mearsheimer and Walt have done silent. They now have their money/house in the Bahamas/whatever. And now, with their retirement amply taken care of, they can allow their academic reputation slide into oblivion. Why else the silence?