Why is there an altitude limit if we have an IDG failure on the A320?
Short answer: This is the highest altitude where the APU can deliver 90 kVA at ISA temperature.



Details

In normal conditions, electrical power is delivered by two engine integrated drive generators (IDG), one per engine.


IDG used both on a IAE V2500 and CFM56-5B engines. source

Each IDG is able to deliver 90 kVA, a power which is sufficient to feed all systems in the aircraft. As electrical power is vital, a second source is provided and each source feeds half of the systems, but is ready to take over and feed all systems in case of failure of the counterpart.

When one IDG is off, the auxiliary power unit (APU) is used as a backup for the second source. The APU delivers both hydraulic power and electrical power.


Honeywell 131-9A APU. Source

At ISA temperature and below 33,500 ft, the APU electrical generator has the same 90 kVA generation capability.


Source: FCOM (3.1.49)

After 33,500 the APU efficiency for electric power generation starts decreasing. If the aircraft was above 33,500 ft and the second IDG failed, this would leave the plane with a single generator (the APU) unable to deliver 90 kVA required by all the systems. So the altitude limit in the SOP.