Dynamics of relation change because "drug" does not work anymore. Or MLCer have more important inner things, doing self. Or it is just inertia, or he is nowhere OW relationship stall and he don't know what to do and he produce a lot damage to self, family spouse and kids. he do not know does spouse want him back, he do not know is it possible to bridge all crap what he done.
A lot things can be. Or all together what I wrote. He can also dump OW and escape & avoid more seeking something else for drug that can be OW.