#2 To make up for his short title reign this year
Rey Mysterio was in the United States Championship picture from WrestleMania 35 till June of this year. In that period, he lost to Samoa Joe at the grandest stage of them all in just over a minute, but his feud continued.
At Money in the Bank, Mysterio ended up winning the United States Championship, but he suffered an injury around that time and he was forced to vacate the title almost immediately, giving it back to Samoa Joe.
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WWE likely had plans for Mysterio as the United States Championship but that didn't work out, so it went from Samoa Joe to Ricochet to AJ Styles, who was the best man to hold the title in a few years.
His reign as United States Champion was good as part of The OC but ultimately, the original plan was for Mysterio to hold the title. We imagine he wouldn't have held it for very long in the first place, but this title win is probably a compensation for his last one that was cut short.