Because of the patriarchal setup. According to the tradition of the Kapoor family, the women of the family are not supposed to work in films. So both Babita and Neetu Singh had to stop acting after their marriage.
Raj Kapoor also believed that filmy bahus don't make good traditional housewives, so he was very much against the marriage of his son to actresses. But Randhir Kapoor and Rishi Kapoor managed to marry with girls of their own choice. But Raj Kapoor put his foot down for the third son Rajeev Kapoor who was not allowed to pursue actresses Padmini Kolhapure and Divya Rana. Raj Kapoor even threatened Padmini to throw her out of Prem Rog if the romance continued. Rajiv Kapoor blamed his father for his personal and professional debacles. Shashi Kapoor’s daughter and wife also worked in films and he produced his wife's film 36 Chowringee Lane, but refused to launch his children. Shammi Kapoor's wife also left films after marriage.
This trend changed with Karishma Kapoor. Babita was a highly ambitious lady and she wanted to see her daughters successful. Also the career of Randhir Kapoor remained patchy due to which they needed financial stability. So Babita at the risk of tainting her marriage with Randhir groomed Karishma to become an actress, and she became the first daughter of Kapoor Khandan to work as an actress. This was followed by Kareena Kapoor and their success opened the gateway for women working in the films. Even Neetu Kapoor made her comeback in 2009 and since then has been working on limited projects. Now even the daughter of Neetu Kapoor, Riddhima has made her debut with her mother in Dadi ki Shadi. And Alia Bhatt is an established actress already.
This tradition was common in many Bollywood families wherein actresses left movies after marriage. Amitabh asked Jaya to stop working in films after marriage at the time when she was at the peak of her career. She returned occasionally to films but sacrificed her larger career. Dilip Kumar wanted Madhubala to leave movies after marriage and the relation couldn't sustain. Sunil Dutt was adamant that no women of her family should work in films. So Nargis Dutt left movies after marriage even though she was perhaps a bigger star than Sunil Dutt at that moment. She only returned to do one film for her brother and won a National award for that. And Sanjay Dutt continued this tradition of not allowing his wives (Richa Sharma and Manyata) to work in films and even when his daughter Trishala wanted to work in films he didn't let her.