May be a silly question but why doesn't Comcast offer stock to get 21CF?
That is a really difficult question to answer simply. However, at its most basic, the primary suitor for a company generally offers stock because they are the preferred outcome. If you are selling, and your preferred outcome is A, generally you like to own shares of A when all is said and done because you, as the seller, think that A is the best fit and has the most potential to make money by buying your company.
The secondary suitor, who is not such a good fit in your opinion and therefore has less upside potential once the deal closes, tends to offer more in the way of cash. Cash lets you, the seller, decide where to invest. You can choose to invest in the purchaser if you think the potential is still there, or you can choose to take your proceeds and invest elsewhere without first having to sell the purchaser stock.
So at some basic level, it's a matter of expectations and perspective. Now tied in to those expectations and perspectives are dozens if not hundreds of factors that people like me are paid to analyze. There is also, from the buyer's perspective, reasons to offer stock and not cash or cash and not stock. Apple, for example, sits on a massive pile of cash. Why would they trade their stock for an acquisition at face value when they already have more cash than they know how to invest? Comcast, of course, does not have this problem. They have a mountain of debt and will have an even bigger mountain if this deal goes through, but it's possible they are aware Murdoch does not value their stock they same way he values Disney. Or it could be they simply don't own the shares to offer, they way Disney does, and would have to buy back the shares before trading them to Murdoch, and it's simply easier to do cash (this is probably a significant part of the reason, Comcast's float is much closer to their shares outstanding than Disney's percentage wise, meaning Disney owns more of itself than Comcast does. Companies like to own themselves, it allows them to benefit most when the stock goes up).
Anyway, despite my efforts, this is getting long. It's complicated. There are reasons both the buyers and sellers choose to offer or prefer to receive cash or stock and you can fill and entire semester of coursework learning the basics of acquisitions and mergers.