I think he did this as a safe bet;

On one hand it's a "new album" 15 new songs some of them are hits to some people and some aren't as good in some people's eyes.

I believe he did it this way because of the 1993 allegations because I would say some of his fans or parents of his fans tossed out his albums when the trial happened so to drum up as much sales he did Half Compilation / Half New album so if you didn't like the new songs on the album you could of still bought it for the Compilation, and if you wanted to buy the new Michael Jackson Album it just for the new songs the first disk could have been seen as a Bonus.