Reason behind the Tour Down Under "People's Choice Classic" 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)
It's complicated, and this may not be all there is to it.

From my reading of part 2 of the UCI WorldTour regulations (p43-44) is


The duration of new "Continental" events is 5 days, unless the event has an exemption from the committee. Existing events have the same duration they had in 2004. The Tour Down Under went for 6 days in 2004. 
A prologue can be included if


It does not exceed 8km.  
It is time trial.
It counts as a race day.



This all confirms your suspicion that it's due to the rules. The Peoples Classic was 50km, and was not a time trial, so it couldn't be part of the TDU. As you say, it was good for attendance and marketing (and publicity).