Because let's face it a person can't change overnight and become dumb and we know Ranaji has always been perceptive, a man who can notice a small look, who can figure out an attack on his wife by some gunde wasn't a coincidence and goes to kunwars to insinuate that he is well aware of that, he willingly freed the man who attempted to kill him at Kunwar's order, because he knew Kunwar was the one who asked him to do that!! he did it willingly, the guy was no dumb!!! they are doing this to him in this track to make it work dramatically though I think there are other ways to have their track without showing Ranaji in such a bad light!!
Unless, and this is what I think or want to think, they want to show us that he is rather in denial mode, he wants to play dumb other than believeing his BRM did this because let's face it even when Jhumki confessed and before BRM starts her drama, he had tears in his eyes and he was hurt and couldn't believe it, it's not about logic or not, he doesn't want to believe she is culprit and to stay in his illusion he can believe any thing, he can believe his wife is insane, he can believe that with agnipariksha his BRM is absolved! he is willingly ready to overlook any other thing! HE IS BEING AN EMOTIONAL FOOL, he doesn't want to face the truth, he wants to believe his heart only and not his brains, his heart is so much attached to BRM, not only he loves her, he just can't grasp ki how the woman who took care of him during all his childhood, who showered him with affection, left the palace and setteled in the jungle when he left for war, the woman who bestowed so much love on him and gave him advice whenever he needed it and seemed so genuine, how can her love be fake!! I think he is even overlooking her weird behavior and her suscpicious histrionics and looks because he has that image of the wise, loving grandma in his mind and heart both and he doesn't want it to get shattered!
If this avi is any indication of how he sees BRM then just check it, not for a nano second she seemed fake here!!

Here I tried to see things from Ranaji's angle, but obviously for viewers he is bound to look dumb as he didn't even try to ask himself who taught fake Sula the mannerism of real Sula, though we know whoever did didn't have to do much since fake Sula was presented as being amnestic, so they could have easily said that whatever changed in her was because of her being amnestic and staying with Gypsies for 6 years, she could have easily adapted to their way of life and became like them!!
In his denial mode, and love for BRM, Ranaji is comitting an injustice with Jhumki, yes he is an emotional fool but he is the most here, because simply BRM never showed him that she is manipulative like his Rajmata, she always showed him that she is genuine, wise and that she loves him, so he just believes her above anyone, more even than his mom, so though he is an emotional fool with every person he loves, but he isn't brainless with them, with BRM he is as her game is very purana, rooted in his subsonscious since childhood, she brainwashed him to such an extent that he holds her above any mistake, he just can't imagine she could make any!
I myself as I said was frustrated with Ranaji but I tried to see things differently, I just hope that when Gayatri's life is at stake, he accepts to face the truth and comes out of his denial mode, and even though it might hurt him, he acts strong and investigates even his beloved BRM because if he doesn't he may just lose the woman of his life, his Gayatri! I hope! It would be a golden opportunity for Cvs to restore Ranaji's brains which have been cancelled in his devotion for BRM!
At the moment I consider him in a self imposed dumbness, a denial mode of the obvious because he is emotionally fool and this time more than ever!!