This has nothing to do with pixie dust and everything to do with risk analysis. Money isn't the only factor - at this point in the process, risk also plays a significant factor, and the risk with Comcast is substantially higher. Right now, Disney has regulatory approval, so the risk associated with this deal not going through is very low (for arguments sake, say < 5%). Conversely, with Comcast, the risk is substantially higher. Let's say the risk is 33% that the deal wouldn't go through because of regulatory action. Now let's say Comcast bids $80B to Disney's $71B. The calculus to Fox's shareholders is: do I take the near-sure thing at $71B, or do I take the higher $$$ amount with the risk that I may get nothing at all? Now, if Comcast's bid is substantially better, the calculus may be that the risk is worth it. But if it's marginally better, or even moderately better, Fox shareholders may decide to take the sure thing with Disney and cash out rather than risk getting nothing with Comcast, even if the potential reward is higher.
It's like saying that you have two doors. Behind door #1, there's a 99.9999% chance that there is $10M. Behind door #2 there's a 50% chance that there's $15M. Do you take the almost sure $10M, or do you go for the coin-toss for the $15M? Does that change if the $15M is $50M? Where's that line?
Personal feelings about the impact this deal would have on Disney, If I were a Fox Shareholder, I'd vote Disney unless Comcast came in and put in a significantly higher bid that Disney didn't match, because I would want the sure thing.