I think that's pretty simple. There is no-one sensible available to replace him.
As
@Beamrider has said many times sacking managers/coaches and their teams is not a straightforward financial contract termination. Slot and team we don't retain would need to be paid up or simply kept on the books. No immediate successor would mean paying parallel fees for an interim.
Until FSG are sure Slot will not get us into the UCL they won't act.
I guess it goes:
1. FSG are intent on riding this out, believing Slot will come good - Slot stays
2. FSG doubt but think he'll get us to UCL - Slot goes in the summer
3. FSG doubt but think he'll get us to CL. However, main replacement is available and they fear will be poached - Slot replaced ASAP
4. FSG begin to think he won't make UCL and think there is an interim available that will - Slot replaced by Interim
I'm assuming they are currently wavering between (1) and (2)