Originally Posted by blarg_
It's quite simple, the WWE didn't feel that Rock vs HHH alone was Wrestlemania main event worthy at the time
I highly doubt that, The Rock vs Triple H was the overarching storyline for six months of 2000 and was part of a rivalry that dated back into 1998 linking various elements such as DX, NOD, the Corporation and the Corporate Ministry. It was every bit a main event, especially after HHH's couple of matches against Cactus Jack shot him into uber heel levels - in which The Rock was also feuding with HHH, playing a prominent part getting Mick Foley reinstated, decking HHH with a steel chair at the Rumble, winning the Rumble to get a shot at HHH etc.
For some reason though they'd always planned it to be a fatal four way, the early posters had Jericho on them. That tells you that it wasn't a hastily thrown together idea because they weren't confident of The Rock/HHH drawing.