Presumably as they offer the most convenient connections . Although a fourth service - afternoon service, would connect well to India and west coast of the US, you'd think there'd be sufficient Perth/India demand to support this. Was the the fourth flight pre-COVID?
To have such a f flight to connect to US/India, you'd need a service that did PER-SIN 1225/1745 (to connect to the 1845 BOM flight), which would mean a 0600/1110 SIN-PER service.