windowseatplease, my thought is because the offshore winds can be much greater than those found onshore on the same day, and of varying directions, you would be flying an ILS type approach to a rig then having the circling manoeuvre at the bottom in bad viz/poor weather/completely out of wind on some days. This would put you at increased risk.
The ARA is used so that you can use any direction, ie go overhead and descending outbound before turning back in getting to minima safely (200', 80kts), or straight in with an en route letdown, to minima so pretty much fool proof. Using the Wx radar and DME and RNAV you don't even need the NDB. (most of which I believe dont always work anyway).
Can you imagine calibrating all of those ILS approaches offshore on a regular basis? A logistical nightmare, especially when the equipment would be rig based, and prone to slight misalignments (rigs are metal and subject to movement and expansion/contraction) A small movement at the rig would cause a big discrepancy over the length of the approach path?? Don't know but just throwing ideas in here now. Aircraft doing calibration flights regularly adding to the airspace traffic.