Because Austin was gone, exactly. When Cena is gone, the "money maker" as far as shows goes is the #2 babyface and #1 heel. But technicality Cena is still the number one guy. Austin was still number one, if it wasn't because of the idiot angle with Rikishi he would have returned to fight for the championship on the spot. He did a month later though, once again pulling Rock from the main event spot for him.
That is not why... That was card placement, both of those matches were built as main event matches if anything, Austin/Angle had more TV time leading into SummerSlam. Talk about Unforgiven 01, Vengeance 01, No Way Out 02 when Austin was in the main event and Rock was not... What a stupid freaking reason. Following the Observer is idiot on this one. You go watch the shows and draw your conclusions.
When you compare both of their career in the company at the same time, Rock was more times before Austin in PPV's. Rock almost never had single main event matches, he had to share the spotlight with guys like HHH, Angle, Jericho, Undertaker, Foley and Show...