You're right. Because BSNL uses PtP (point to point) wireless technology for their AirFiber service, it's availability is not that great, unlike Airtel and Jio AirFiber which relies on 5G FWA with greater availability.
It’s only viable in areas where the wired connectivity is unavailable, like @Mr.Bhat mentioned dense forest or villages with no 4G/5G connectivity.