#5 Injury

At 41 years of age, AJ Styles is by no means "young" in the pro wrestling world. Styles has been one of the hardest working Superstars on the roster, wrestling most live events and on SmackDown, and the tough life on the road as well as the pain he inflicts on his body with his acrobatic moves may have taken a toll on his health.
WWE announced following the match against Bryan that Styles was injured and hence will not compete in the MMC match, although this could be a kayfabe injury.
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Styles may have asked for some time off to heal and rest, as well as spend time with his family, hence may have been written off television with an injury. The "injury" also allows him to come back with a bang and get a huge pop later down the road, and perhaps face off against Bryan.