Nikki Sixx told Rolling Stone in 2014 that the band signed a "cessation of touring agreement" and "legally, we can't play again." He told Rolling Stone then that there was a loophole, however:
“The only loophole is if all four band members agreed to do it, we could override our own contract. But we know that will never happen. There are people in this band who will refuse to ever do it again, and you’re talking to one of them. There is no amount of money that would ever make me do it again because I have such pride in how we’re ending it.”
A few weeks ago, the band announced their return in a video that describes the 2014 contract and then shows it explode on a desk.