#3 Samoa Joe wasn't supposed to be champion anyway
Samoa Joe lost the United States championship to Rey Mysterio in controversial fashion at Money in the Bank, as his shoulders were clearly up. His post-match beatdown on Mysterio was essentially to write the legend off, with Mysterio voluntarily vacating the title and handing it back to Samoa Joe.
Had Mysterio not been injured, Joe was not going to be champion anyway. It's possible that WWE would have re-run the Mysterio-Joe match at WWE Stomping Grounds 2019, but the feud likely would have ended there.
Since Samoa Joe was simply handed back the championship, WWE likely felt no real reason to keep the title on him as a placeholder. Hopefully, it means that Joe will eventually step up to the main event scene again, because he deserves it more than Baron Corbin does.