Rick C-137's backstory in the Rick and Morty season 5 finale shows him returning to a dimension where Beth was abandoned by her actual father — but why didn't Rick find a dimension where his wife, Diane, was still alive? The season finale finally explains Rick’s full origin, validating some fan theories and confirming Rick's season 3 backstory lie. However, not all of Rick's decisions after losing Beth and Diane were thoroughly explained, like his decision not to move to a dimension where Diane is still alive, even though he clearly suffered greatly after losing her.
Rick never returns to a dimension where Diane is alive for a variety of reasons. At first, after losing his family, Rick becomes obsessed with revenge. Making it his primary goal to find and kill his family's killer, he removes the universes where the killer Rick wouldn’t be — these are the universes with Diane in them. Later, as he repeatedly fails to find the killer, Rick C-137 becomes jaded, depressed, and nihilistic. He doesn't care what happens next, so he is in no fit state to be a family man again. Eventually, Rick joins another Beth and her family, but never another Diane. Rick's memory of losing his wife is buried deep within his mind, and it would be too painful for him to revisit it, even by returning to her. He would always know it was not C-137 Diane.
Rick Can Never "Complete" His Alternate Universe Family With A New Diane
Rick is not completely at peace with living with a different family. In Rick and Morty season 5, episode 8, “Rickternal Friendshine of the Spotless Mort,” Rick meets a younger version of himself in Birdperson’s mind. When he mentions his grandson Morty, young Rick comments, “You’re one of those creeps who moves in with abandoned adult Beths? You live with a version of our dead daughter.” Rick C-137 clearly has not forgotten about the death of his wife and daughter, and he is aware that the Beth he lives with is just a substitute — which also might explain why he doesn't want to know if Space Beth is the real Beth or not. Rick might find it even harder to live with his wife's substitute than with his adult daughter's.
Rick and Morty is bound to further explore Rick's backstory, especially the creation of the Central Finite Curve, the subset of dimensions in which Rick is the smartest person in the universe. Since Evil Morty scans Rick's brain to understand how to break the Central Finite Curve, it’s implied that he is its creator. It's possible Rick created the Curve in order to narrow down the places where the killer Rick could be hiding. Developing upon this story would also explain more clearly why and when Rick decided to abandon any dreams of returning to another version of his wife. Indeed, even as it's implied in the mid-season finale of Rick and Morty season 6 that the show is giving up on canon and serialized storytelling, it's highly unlikely for Rick to decide it's time to replace Diane.
If Rick Replaces Diane He Loses The Last Of His Humanity
By Rick and Morty season 6, Rick C-137 is so extremely modified that the only original part of him that remains is his love for Diane — the only Dianne — and letting go of this means losing what little humanity Rick has left. He himself admits that he's basically become Inspector Gadget; he's had multiple bodies, transferred his consciousness countless times, hopped universes, and replaced everything in his body with better organics or cybernetics. Rick C-137 couldn't be any further from the man that he originally was, and if Rick shacks up with an alternate Diane, he'll be at a place where he has to acknowledge literally everybody he cares about is replaceable.
In fact, when Rick fixed his portal gun in Rick and Morty season 6, the temptation to replace Diane undoubtedly crossed his mind again — yet his stance on Diane is what anchors Rick to a reality that even he can't change. Without it, there'd be nothing to stop Rick from just wiping out everything to start anew, which he could do in a heartbeat. Just as Rick keeps a family of individuals from a hodgepodge of universes to remember his history and own up to his actions, Rick will never replace Diane because her memory is what makes him who he is.