#2 To guarantee a hot crowd for the main show
It is no secret that the fans in attendance can either make or break the show. When the 4 NXT superstars made their main roster debut, the crowd was totally dead. Fans like me, who were watching the episode on TV, were not hyped up for the NXT superstars' debuts and the Twitterati was not happy with the crowd.
With the NXT superstars Ricochet and Aleister Black set to make their first pay-per-view appearance, WWE wants the crowd to be hot for their match. The chances of the RAW tag team championship match being the first on the main show are high and thus, WWE does not want a dead reception for these superstars.
Andrade and Mysterio could get the crowd going with their terrific performance. Maybe that's why they are on the kick-off show.