Since it had been the biggest story all year, why not have the match on your biggest show instead of doing it when they did?
You can either do Austin winning the belt at St Valentine's day massacre or you keep the mankind vs rock match with Rock winning thanks to a debuting Big show. This sets up Mankind vs Show at mania, Austin vs McMahon, and say Rock vs Taker.
Really though all that fantasy booking aside, what was the reason for not doing Austin vs Mcmahon at mania?
Because that would be a colossal waste of Steve Austin at Wrestlemania. No one buys Vince as having a chance against Austin without the deck being ridiculously stacked, and Rock acting as Vince's avatar was good enough to draw a giant buyrate anyway. Doing Vince v. Austin as a real match was fine for a throwaway show in February, but for the biggest of the year? No chance.