Because all the regulators must approve the acquisition, if one of them says no there's no acquisition. MS will appeal the CMA decision. If MS doesn't win that appealing, the acquisition will be cancelled.
And on top of that, they have a milestone to complete the acquisition before July 18th. If they don't have the acquisition completed by this day, the acquisition would also be cancelled and MS would have to pay them I think it was $3B more as compensation to the ABK shareholders (mainly Kotick). But they can decide to extend the milestone, so that last part pretty likely wouldn't be the end.
There's a certain English man in USA that is going to be very happy once MS loses their appeal vs the CMA and the acquisition gets finally buried: