What disadvantages would it have brought to airlines or anyone else if the LHR curfew were extended to 07:00 while the airport is so little used? Is it just they don't want residents of West London to get used to sleeping with the windows open?
The argument that's usually advanced by the airlines is that it would cause problems at the origins of the early-morning arrivals.
For example, the first arrivals this morning were from Hong Kong and Singapore. BA032 is due to depart HKG at 23:10 local time and arrive at LHR 04:50 (it landed just after 04:30).
Delaying the arrival of such flights at LHR by 1-2 hours would obviously necessitate delaying the departure from HKG/SIN/KUL/etc by a similar amount, which the airlines argue would be commercially unacceptable (and might also infringe night restrictions at some originating airports).
So that's why West London (and Windsor) residents continue to be woken from 04:30 onwards.