I just came back from Kolkata to BKK last evening on Bhutan Airlines, which had the best time of day departures in each direction. Note how those Lion Air flights happen in the middle of the night. Some people can do that, not me. Also, India does not encourage tourism it seems. In 10 days I saw about 10 western people, in a city of millions. The airport was almost empty coming and going, so yes they want business. But flights at normal times would help. I will say that Kolkata is definitely worth a visit, and the people were mostly wonderful to me.