As to "why" Fox is looking to sell: like many complicated things, there's not really one, simple answer. A significant part of the answer, though, are the Murdochs. Even though Fox is a public company, Fox is a family business insofar as they're concerned (they own about 17% of Fox, but about one-third of the preferential voting rights). Rupert Murdoch has made efforts to pass management down to his kids in recent years and they've taken over parts of the company. There's been speculation, however, that Fox is falling behind. One of its major investors unexpectedly sold off his interests recently. When the story broke that Fox was shopping itself, the reason was that Fox didn't have the scale to compete on the big stage (which reason the Murdochs publicly dismissed).
If I had to guess, a driving factor is that this is the best time for the Murdochs to cash out. They could make Fox bigger and more competitive on its own, but they'd need to bring in outside money to do that. That probably dilutes both their financial interests in and control over the company. If they sell, they lose the company but make an awful lot of money in the process. In the latter scenario, they at least get a pay-out while they still call the shots at Fox (and if they can parlay that into management positions at Disney post-merger, they may just make themselves even richer).
If I had to guess, a driving factor is that this is the best time for the Murdochs to cash out. They could make Fox bigger and more competitive on its own, but they'd need to bring in outside money to do that. That probably dilutes both their financial interests in and control over the company. If they sell, they lose the company but make an awful lot of money in the process. In the latter scenario, they at least get a pay-out while they still call the shots at Fox (and if they can parlay that into management positions at Disney post-merger, they may just make themselves even richer).