1. His contract
Wayne Rooney currently has two years to run on his deal at Old Trafford, and it's worth a figure believed to be £250,000 per week. That's a huge sum, and given the way United have acted over the player since signing it, it seems that the club isn't really convinced that it offers value for money. Two years is kind of the do-or-die moment with contracts of players that have a few years left in them - when it reaches one year, the value will significantly be reduced. Now, United won't get a colossal amount of money for Rooney - £30m would be a good price - but if they're going to sell, they'll do it now. And they will do it now, because the alternative is a new contract. There's no way Rooney would accept his wages being slashed, and there's no way United would offer him a deal comparable to his current one.
2. Sir Alex Ferguson
Anybody believing that Ferguson will simply take a step back out of the spotlight and give David Moyes a completely free hand is naive in the extreme. Ferguson clearly thought Rooney ought to leave Old Trafford - he simply does not subject players he wants to keep around to such vociferous public criticism - and his influence within the club and on Moyes will still be huge.
3. David Moyes
Rooney and Moyes have supposedly patched up their differences, but their squabbles go beyond a lawsuit over the contents of an autobiography. In Rooney's book he also mentions a typical example: a press conference after he signed his first professional contract, in which he was sipping from a bottle of water. Beside him, Moyes growled quietly "Use the f***ing glass!" in exasperation. That's not the behaviour of two great friends, it's the behaviour of two people who rub each other up the wrong way, like an old and bitter marriage.
4. Other players
Rooney is unlikely to displace Robin van Persie from the United lineup anytime soon, and with Danny Welbeck, Javier Hernandez, and Shinji Kagawa competing for the front two central spots and all potentially showing they have more to offer there than Rooney, it's highly debatable as to whether Rooney even gets in United's strongest team. If United sell Rooney, they don't need to buy a direct replacement, because any combination of those four works out just fine, without even taking into account the likes of Wilfried Zaha and any other potential arrivals such as Thiago or Cristiano Ronaldo.
5. His attitude
When Wayne Rooney was undergoing some of his worst form, after his miraculous year following Ronaldo's departure, one of the worst things about watching United was that as well as being terrible, he also seemed undroppable. When he entered another dip last year, Ferguson was far less shy about leaving him out of big games, and the two appeared to fall out over it. Ferguson also hinted that Rooney had fitness problems, and it seems like the club have reached the end of their tether.
6. His attraction to other clubs
Chelsea, Manchester City, Real Madrid, Paris Saint-Germain, Bayern Munich, Juventus, Arsenal and Tottenham Hotspur are just some of the European clubs looking to spend big money on a striker this year. What is making this summer such an interesting one is that there are fewer targets than there are clubs, especially with Falcao having joined Monaco. Edinson Cavani can only join one of Real, Chelsea and City, so even if nobody's willing to make a huge bid right now, they may well be later in the window. For the sort of clubs where money is no object, Rooney is a hugely attractive player and it's hard to imagine nobody wanting to take a chance on him.
7. Because the club want him out
After the revelation of Rooney's second transfer request, his camp appeared to desperately backtrack, issuing denials, while Alex Ferguson and Manchester United stuck to their guns. All the noises coming from Ferguson suggested that Rooney's days were numbered - try to imagine the last time he said anything like "if he was fit and in-form then he wouldn't be left out of the team" about one of his stars.