It is basically a change in the perception of cinema. When India started making films performing in public was considered a dishonourable job. Infact in theatre plays there was a category of male cross dressers who used to perform female roles as well. And this happened with our initial films as well like Raja Harishchandra where male played female parts.
Soon female characters were played by women but there were largely 3–4 types of women who worked in such film
Hindu families who were part of the performative traditions, temple performers or Devadasis. This prominently includes the Maratha Gomantak Samaj from where we got actresses like Nanda, Padmini Kolhapure, Shilpa Shirodkar, Kimi Katkar etc. From dancing background we had Padmini Ramachandran, Vaijyantimala, Waheeda Rehman. Hema Malini etc.
Tawaif tradition among Muslims (Begum Akhthar, Nargis etc)
Anglo Indian women (They were prominent during the silemt era films because one they didnt need to speak dialogues and secondly they were open to perform bol scenes in movies. After the talkies cinema such actressses like Sulochna, Premila lost work because they couldnt speak hindi properly. There were some who persisted like Fearless Nadia)
Actresses from poor/desperate/greedy wgo used films to earn quick bucks for their family (often forcing their daughters to work)- Daisy and Honey Irani, Naaz, Sarika, Neetu Singh, Meena Kumari, Madhubala, Mandakini etc.
Actresses from film families- there is a long list of such actresses from film families.
From late 1950s onwards we see a new crop of female actresses who took training in acting. It started with actresses like Sadhna and Asha Parekh and strengthened with the establishement of NSD and FTII. Shabana Azmi, Smita Patil, Jaya Bhaduri etc come from this background
Now what has happened in the 21st century is due to expansion of media and multiplex era cinema is no longer located only in Mumbai. With the proliferation of OTT and other mediums, films anf content are made everywhere and more number of women are entrring the space. The traditional dominance of certain families or groups have withered down and cinema has become corporate. Actressses are coming from all social groups.
Also cinema is seen as a profession and a good vocation for money and fame so the erstwhile social sanctions on public performance has so much eroded that today every other woman can be a creator on social media which also has emerged as an avenue for media attention, financial freedom and even road to cinema. Also corporate culture has standardised the working of cinema with a lot of whims and fancies and idiosyncracies of old system replaced.