A direct route is not commercially viable perhaps. At least not year round. And to route to BKK is also not logical when QR can get Fifth Freedom traffic through somewhere else eg KUL. Hence you get it as a one stop.

Same situation to Bali-Denpasar which is routed via SIN. And also to CEB where it was also once routed via SIN. The latter route no longer operates.

QR has on some days 3x a day from DOH to KUL in the peak summer months. So KUL is more commercially lucrative than to HKT for the DOH / Middle east market than HKT is.