The Tour Down Under (TDU) is technically a 6-stage race, but the day before is the "People's Choice Classic",
The People's Choice Classic, also known as the Down Under Classic, is a single day road cycling race held in Adelaide, South Australia, Australia. The race is the official warm-up for the Tour Down Under. Founded in 2006, the race exists as a men's only competition
The race is the day before Stage 1, but it doesn't have any bearing on the Tour Down Under, and riders don't earn UCI points.
Is there any specific reason this is not part of the TDU?
My first thought was it's good for attendance/marketing/etc for "the race" to both start and finish on a weekend (and both take place very near the city-centre), but there might be some UCI rule preventing a 7-stage.. but this theory is completely unfounded (I couldn't find anything to this effect in the UCI rules/race-guidelines)