They actually explained this recently when they announced their film slate for the next 3-4 years. Since Disney and Pixar are one in the same, they will use their summer films to promote their holiday films and their holiday films to promote their summer films. So, the advertisement that was attached to Wall-E was Bolt, because it is being released in November.
In the future, it will generally go Pixar-Summer and Disney-holiday, but, Pixar will also be releasing a holiday film one year, so, this isn't something set in stone. I am sure that the company will like to switch it up in their long-term future, but for now, this is the format they will follow.
We should see a full-length theatrical trailer for UP attached to BOLT. I am looking forward to BOLT and the trailer in November.
Hope this helps! :wave: