Apron plan is to expand north in first phase, which is across the current staff carpark and care-hire area. With car park already full to overflowing in summer they can't easily build over those areas without an alternative.
Not sure if that's the main hold up but may be partly to blame along with funding. Expanding apron east would be next but requires moving the fuel farm and major earthworks.
The current land-side parking set up isn't optimal for set down, bus, pedestrian or car hire access to the new terminal. The re-design should have happened along with the terminal building, but was held up by planning objections which were overruled by ABP last year.
Design involved a new long term car park with covered walkways along airport road, a new roundabout and entrance from existing road next to the new terminal, with bus bays and set down points outside the departure lounge. Purpose built car-hire compounds would also be built on current overflow car park.