Because he strongly believes that Twinkle will bring in good fortune for his family. She's like his own personal incarnation of Goddess Laxmi. Also, correct me if I am wrong here, but Guruji has also said that TwiNj's horoscopes were matched really strongly with one another and bringing Twinkle into the house will basically do wonders for them. Since Manohar listens to everything that Guruji says, he's really stubborn in making sure the wedding happens.
Again, correct me if I am wrong here. I didn't get to see much of the initial episodes so I might be a bit off.