Why establish someone as 'The Guy' at a midcard Pay-Per View? Simply because Summerslam, The Royal Rumble and Survivor Series are three of 'The Big Four' doesn't mean having someone established there will make a big impact. Look at The Rock & The Undertaker, both debuted at Survivor Series; yet were't Established until a lot later.
Wrestlemania is WWE's most watched ppv. It's the ppv that even people who don't watch wrestling would check out.
Wrestlemania is the most watch ppv, so more people will see the new top star be made.
Cz that's the grandest stage of em all
creative differences.