I would guess that the whole "remaining inside CAS" argument is the cause. On top of that, no radar service can be given below FL70 so it'll be a basic, at which point pilot says they cannot accept the basic service........ Just not worth it, if the flightplan has been filed to remain inside CAS then give the appropriate directs which may be different to those pilots are used to.
REVTU - MAY into EGKK is another hit and miss route now too, some TC controllers will take them descending FL200 and offer the deconfliction service if necessary, some will not, some aircraft won't descend outside CAS and they then end up high and fast.....