Microsoft just let it slip why Call of Duty is not on the Xbox Game Pass - Sony doesn't want it there.
According to a footnote in Microsoft's recent submission to the Competition and Markets Authority, UK's regulatory body when it comes to these sorts of things, Microsoft reiterated, "existing contractual obligations that Activision Blizzard may have with other platforms" will continue as is after the $68 billion acquisition pushes through.
But, while this is good news for Sony, Microsoft is using this to its advantage. Specifically, Microsoft writes that this will include the agreement "between Activision Blizzard and Sony" wherein the deal restricts the "ability of Activision Blizzard to place COD titles on Game Pass for a number of years."
Unfortunately, Microsoft didn't elaborate further, but it doesn't take an expert to know what Microsoft meant.