This has been a long-running one Karl, and we are no nearer the real reasons to this day. Until 1989 they did a different set every night, everything was interchangeable. There were some shocking performances of songs not rehearsed for years!
It seems they were desperately unhappy with how hit and miss the live show was, so for the 1989 US Tour they decided to rehearse 20 songs, mixing them up as two distinct sets that were rotated every night. On occasion, some other songs found their way into the set, but these were few and far between. The key for Hooky was the lack of professionalism they believed they were showing.
The next set of gigs in 1993 featured the same set every night bar two, one that featured a shoddy take on Special, and one that was shorter than the rest.
In 2001/2002 they again stuck to the same set every night pattern by and large, agreeing a set for a series of dates and not really varying it. The Australian dates offered a choice between Love Vigilantes and Ceremony, that was about all.
Since Bernard adopted an autocue to provide the lyrics, and Phil joined the band, there have been fewer set changes than before. They also upgraded their sequencer technology several times between 1989 and 2002, which meant that a lot of the songs they used to play got left behind in terms of sequencing. They hired Roger Lyons of Lionrock to sequence some of the newer songs, and he ran out of time sequencing older material for the live set.
One thing is for certain, we will never see the old days again where the set was changed every night. Let's be grateful we still have the chance to get to see them at all though!