Short answer: Vince's 'Austinless insecurity'
The WM 2000 main event that almost happened was Jericho Vs HHH vs Big Show Vs Rock (there are still promo images circulating)
If you want to understand why we got an overbooked main event for Wrestle Mania 2000, just look at Unforgiven 1999's main event (with Austin as enforcer no less despite being injured.) Unforgiven 1999 was the first PPV main event without Stone Cold main eventing since No Way Out 1998. And Vince felt such a lack of star power, he made it a SIX WAY MATCH. There were some big names in there, but not enough in Vince's eyes.
So there was no trusting The Rock, or HHH enough to carry a singles match without Stone Cold. This is further proven by the booking of Backlash, Judgment Day, and KOTR 2000 main events which all featured some combination of Stone Cold, Undertaker, Shawn Michaels, Kane, or the McMahons. Fully Loaded was touted as a 'triple main event' to help sell The Rock/Benoit main event. SummerSlam, and Unforgiven also multi-man main event matches. I don't think Vince truly believed The Rock was on Austin's level until No Mercy 2000 when it was a straight one on one match -- Rock Vs Kurt Angle. No Mercy could've been billed as a triple main event as well (Rock/Angle, HHH/Benoit, Austin/Rikishi.) However, by this time Vince had Austin back, and there was no need in his mind.
Compare The Rock's 2000 run as a face versus Austin's 1998-1999 run. Notice how the marketing, and multi-man matches are not there? It was mostly one on one matches with rare exceptions: Breakdown 98 where Austin lost the belt due to a double pin. KOTR 99 with a 2-on-1 ladder match, and the SummerSlam 99 triple threat (depending on who you believe Mankind was added to the match either because Jesse Ventura didn't wanna raise a heel's hand, Austin needed someone else to carry the load due to his bad knees, or Austin didn't wanna job to HHH.)
