Hi Aero Tech,
I'm currently flying 767s for a UK based JAR-OPS 1 operator (I suspect the same as kenparry used to work for) and The MEL allows ETOPS up to 138 mins provided both eng driven genis(IDGs) are working & the Hydraulic Motor Geni (HMG) is verified to operate normally.
As to why, I think it's down to what you have left if both IDGs should fail & the simple answer is not a lot!! Lose 1 IDG & not much happens; there's obviously some load shedding (galleys, pax convenience items, the odd hydraulic pump etc), but the working IDG will still provide a full set of flight deck instruments.
The HMG is powered by the centre hydraulics & will start automatically- hopefully! - when both L & R main AC busses are unpowered. The only busses that are now powered are:
AC
1. L & R Inst Transfer Busses (powering items considered necessary for ETOPS),
2. Capt's Flt Inst Transfer Bus,
3. AC Standby Bus.
DC
1. Hot Batt Bus,
2. Batt Bus,
3. DC Standby Bus.
This basically means that the Capt has a few, essential flight inst & the co-pilot has none!
Have no knowledge of the A330, but if your claim is correct maybe the equivalent of the HMG is cleverer & provides a greater level of power.
Hope this helps.