#1 Because they're trying to save Nakamura's title reign

Shinsuke Nakamura captured the Intercontinental Championship at Extreme Rules on July 14 this year and his reign hasn't exactly set the world on fire. Despite being champion, Nakamura has often been overlooked on the blue brand and hasn't had many major feuds over the past two months with the prestigious title.
This isn't too different from Nakamura's reign as United States Champion last year, as though the King of Strong Style held the title for several months, his stature in the company did not change at all during or post his title reign. Having Nakamura retain tonight may have been because the WWE are trying to save face with the Asian Superstar.
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After last year's underwhelming run with the title, the last thing WWE wants is for fans to associate Nakamura with disappointing title reigns. Instead, having him retain tonight allows WWE at least one more month to make the former Royal Rumble winner seem relevant once again, as well as help rebuild some of the legitimacy of the IC title, as the champion should be considered the 2nd top star on SmackDown Live.