Not sure about everyone else's reasons but I can tell you from our perspective.
The Benn Hall is the only big, decent venue in Rugby and thus is wanted by everyone. This meant we had to be out by 6am for the next ones in to set up.
We wanted to still give 10 hours, so had to start 8pm.
We also had brilliant backing for the last five years from the soul nite crowd and would not turn our backs on them.
When we had our trial all-nighter Feb '07, the management were so impressed, they agreed straight away to start 2008 with all all-nighters, the only hiccup there was that April and Sept already had people coming in at 6am!!!
So, rather than not running those two, we went for 5am finish...still 9 solid hours of dancing...