This issue comes up frequently even in discussions, not specifically about airlines, exclusively. All of a sudden, now that T3 at DEL has been built, people are dreaming of the Airport becoming a transit hub for Asia-India-EU/NA traffic overnight.
Here is a brief summary covering must of the points I have seen being raised.

Indian almost 2/3rds of the international traffic pie is eaten by airlines like QR, EK, EY etc leaving people wondering why?

For decades, air travellers made up a small fraction of Indians. India was growing at well, the Indian rate of growth, any changes in the market were incremental. The status quo of Indian aviation market, the biggest chunk of which was Indian-Europe(mostly London)-NA was a combination of AI and European carriers, along with PanAm and TWA. Asian market was shared between Indian and flag carriers of Thailand, Singapore, Japan, etc, Every one was happy in the status quo.

As air travel picked up in volume, the 60's and 70's --- yes, we do need to go back need to go back that far --- another market began to develop. It was to the Middle East, made up of working class Indian labourers. That market was exclusively served my Middle Eastern and Indian, in reality, only one, despite IA and IC, carriers.
As now, there were those who looked down upon Desi in comparison with "phoren" European (read brown and white).
There was a division of classes too. The poor workers going primarily to the Middle East, and the more affluent ones going to Europe and Asia.
Essentially, there was a clean division among various players.
As PanAm and TWA began to shrink (around the world) beginning with the mid to late 80's, the lack was taken over by European carriers every where and the likes of Delta, AA, in the U.S.

For all we now, things would have stayed in that mode until today, had it not been for the revolution, most visible aspect of of which was the IT robots from India.

PanAm was already gone, having been canibalised by Delta and United. TWA, after starving for years, had been digested by American when you began to see Indian IT robots jetting across the United states.
All of a sudden, India-U.S. market serving passengers originating in India exploded. India-U.S. market was overheated, making it difficult to buy tickets to India at a short notice. In the 80's and even up to the mid 90's one could buy a ticket, a day before travel, and not pay through through one's nose, using consolidators.

When the India, U.S. traffic exploded, it wasn't just that market that was growing. The tigers of Asia, and other Asia Pacific traffic, mostly to the U.S., the biggest consumer, were and had been growing too.

Traffic origination in other countries was able to support multiple carriers. Indian markets was essentially a captive of only one Indian carrier (IA and IC were essentially one even though they operated as separate entities). By the time Jet and other Indian carriers entered the market, it was too late. All growth had been absorbed by those who were in the right place at the right time, having made right and insightful decisions before the expanding markets descended upon us.

had Indian carriers been in the market, as it was developing, they could have competed with the others. They have to work much harder to take a piece of the pie away from those who are already fattened and ready to defend their share.