Because US DoJ approval isn't anywhere near the end of the process. You still need approval in other jurisdictions, and you still need both companies shareholders to vote (Comcast wouldn't need their shareholders to vote if they remain all-cash, only if they add stock to the offer).Seriously, hasn't the Fox shareholders vote on the Disney/Fox deal yet? Why did Comcast even bother to bid for the deal in the first place if the Disney/Fox deal is close to be approved?
The DoJ process has gotten way overrated here; Murdoch hasn't even rescheduled the Fox vote, so the bidding window is still open.
I think Murdoch will wait as long as plausible (at least another week or two) for Comcast to make a counteroffer. Why? Because it makes him richer.