Why there are no direct flights between India & Australia?

It's precisely because of the distance that flights from India to Australia required to have a stopover, and a change of flight crew. The Qantas flights from Mumbai to Sydney take around 14.30 hours, and that's longer than an international pilot is allowed to fly. So Qantas breaks the trip at Singapore where there is a changeover of crew. The same flight then continues on to Sydney. The same applies to other airlines doing the India to Australia route.

Qantas used to fly directly to Mumbai a couple of times a week but when the GFC hit they canned a lot of their flights. Obviously people didn't want to go to India so that was one of the routes that got cancelled. The students and migrants tend to just take one-way flights and there's not much money in that for Qantas, they need business travelers to fill their planes on a regular basis.

I've flown direct to Mumbai via British Airways - but that was several years ago.

Having flown that flight & having spent time in India I can see why flights have dropped off. They would get very little repeat custom from Australians!!!!!

I suggest asking air India would be a better option. It's not the responsibility of Australian airlines.