A quick google suggests that there is an air service agreement in place between India and Singapore that limits how many seats may be operated in total and by each airline. Even Scoot are moaning below about insufficient allocations.

So it's possible that the likes of QR may not be allowed any allocation to sell, unless the agreement is bypassed by constructing the itinerary from separate tickets or using points. Separate tickets under one PNR may be possible through a wise travel agent if QR won't do it, but I imagine it starts getting very pricey that way and/or requires a stopover in Qatar, at which point a direct flight becomes better value

https://www.deccanherald.com/busines...eement-2757929