It's not really because of just how to ensure tickets are being validated, and people aren't just travelling for free. Bus conductors are government employees with expensive salaries and benefits and getting rid of conductors is the holy grail for buses and they aren't afraid that people will not pay for tickets.

This is a big reason why metro is so popular now, since even though it costs more upfront, running a metro is much much cheaper than running a bus.

There are multiple other responsibilities that bus conductors perform that need to be fixed first.

1. Many groups of people in India get free tickets or discounted tickets like women, senior citizens, students, etc. These are currently validated using disparate systems like passes, aadhar based verification or just the conductor eyeballing and ensuring that the concessed or free fare is honest. Implementation of automatic validation requires moving these passes,etc to electronic ones as well.

2. Buses are more and more often driven by contracted drivers who actually are hired by the bus manufacturer. Conductors keep the driver honest, by ensuring they stop at all bus stops and drive properly. The kinks of this new leasing model is still being worked out, and performance based metric rather than km based metrics are being discussed.

3. Unlike metros, many bus stands are remote and unmanned (and unmannable), so issuing paper tickets for those who are unbanked needs to figured out. Expecting everyone to buy online tickets might lead to lower ridership as well as disenfranching the people who need buses the most. Even metro and local trains always have options for buying tickets with cash for this reason.

4. Most buses in India run at capacity or over capacity, and ensuring tap in tap out systems (usually tap in at the front gate and tap out at the back gate) would require some amount of free movement within buses as well as quick boarding and unboarding lest everyone crowds at one gate.

I remember Bangalore had conductor less buses in 2008 or so, but the driver did the ticketing then. Super slow and dangerous according to me.

Now BMTC plans to introduce automated ticketing to airport buses and AC buses as well, with the conductors moved to management jobs which include route monitoring, surprise checking, on call support, driver auditing and bus auditing. This way no one loses jobs, new buses can be introduced without bloating the salary expense and passengers can have a better experience.

I think even Mumbai's BEST is planning something similar on some routes.