Is that not due to SIA internal requirements for an FO to have a number of PF sectors before being upgraded.
Many FOs chose the route of higher pay on long haul aircraft getting few sectors resulting in a long drawn out time before being eligible fir a command upgrade.
My understanding one of factors to be eligible they ask for is 500 PF sectors, which many would say is even insufficient to handle a transport category aircraft to its limits.
There are many FOs that have reached the requirements! The reason for the delay now is the excess capts. The current excess is about 60+ Capts.