Passengers prefer frequency over aircraft size. DEL-BOM is like JFK-LAX. If capacity allows, airlines should (and do) fly 30 daily A320 instead of 10 daily A380. Fuel consumption of A320neo is lower (esp if you can push utilisation higher) so that there's no real reason to use A380.

Runway capacity also isn't really, A320s (and B737) can be spaced very closely on approach and departure, up to a third of that of a A380.

Beijing to Guangzhou has 380 service via China Southern. Beijing to shanghai isn’t that far, and is better served by more-often service to compete with high speed trains that are also viable.

China Eastern even has an express desk for shanghai flights and they are given priority to prevent all too common Beijing delays.

AFAIK Delhi-Mumbai is served almost completely by A320s and 737s. I doubt most people traveling on that route could afford a 787 or A380 flight.

Cost isn't the only issue, the A380 is probably slightly cheaper overall (a few dollars/ticket - assuming you fly the and maintain the airplane on the most economical replacement schedules), but it flies each leg half as often to reach that (presumably you have either half as many large planes, or the large plane covers two routes instead of one) which means customers don't have as much convenience in choose when they want to fly.

If you have 400 people every 15 minutes wanting to make a flight, then a lot of A380s makes sense. However if it is 400 people per day you can make them happier by running 2 200 passenger airplanes one for those who want to leave in the morning, one for those who want to leave in the afternoon. (and 100 passenger planes might make even more sense running 4 times a day, down to 25 passenger planes running hourly)

More smaller planes also allows more routing flexibility.

These are very complex decisions that each airline has too make.

Isn't the whole point of the A380 that it's cheaper per passenger-mile when fully loaded up? Why would an A320 ticket be cheaper? Is it the cost of the airframe? Could better leasing options (as other airlines leases run out) solve that?

I just assumed that buying a new jumbo jet would need costlier tickets, the initial cost more than offsetting the cheaper operating costs. Otherwise why are they not operating yet?

It's a complex equation. An old enough plane will have higher maintenance costs. And many planes are leased instead of bought outright and those lease prices already price in depreciation and expected lifetime.