If all delayed flights were forced to abide by the 21:30-06:30 curfew, then it just makes matters worse. If a delayed flight is forced to divert to BFS, then obviously that aircraft will need to position itself at BHD before its morning departure. No flights are allowed to arrive before 06:30, meaning there is no chance that an aircraft diverted to BFS the night before would be ready for one of the airport's 06:30-07:00 departures. Consequently these flights are delayed, causing further delays throughout the day, and consequently leading to the aircraft being diverted to BFS again. Think of the time and money wasted by airlines and airports for diverting all delayed flights.
In addition, the vast majority of local residents moved to the area after BHD recommenced commercial flights in 1983. These people actively chose to live in close proximity to BHD, so they have absolutely no right to complain about aircraft noise. If they didn't want to be disturbed by aircraft, they shouldn't have moved to the area - it's as simple as that. BHD made the right call by allowing delayed flights to land at the airport past the curfew.