#1 To keep Roman Reigns away from the WWE Championship

The major reason why Vince McMahon has decided to prolong this feud is that he does not want the WWE Universe to boo Roman Reigns once again. He has kept him from the title picture ever since his return only to get the WWE Universe cheer for the Big Dog.
Had Roman Reigns pinned Shane McMahon fair and square at WWE Super Showdown, he would have run out of opponents on the main roster. This would have forced the writers to move him back to the title picture.
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However, WWE did not commit the mistake they had made in the past. By allowing Shane to emerge victorious at Jeddah, Vince McMahon has all but confirmed that Roman Reigns will not feature in title feuds at least until Extreme Rules.
Thus, these were the 3 reasons why Vince McMahon allowed his son, Shane to end Roman Reigns' 6-match undefeated PPV match at WWE Super Showdown.