There are a number of reasons and when one says Man Utd will not win this season's Premiership one better have a myriad of good reasons.
1) A number of teams got better: You have to single out Liverpool and Spurs. My feeling is that Fernando Torres and Ryan Babel are going to be the deal makers for Liverpool this season. That is because Torres and Babel were coaching decisions, unlike Andrei Shevchenko who was solely brought over at the behest of Roman Abramovich and became the focal point of the unseemly row between Mourinho and Abramovich. Spurs got Bent, Kaboul, and Gareth Bale, to bolster an already potent strike force. Remember Dimitar Berbatov was a huge Man Utd target. Chelsea has not lost a step and Pizzaro and Malouda open a more innovative angle on the conservative attack, largely centered around Drogba. Arsenal had Man Utd's number last season and there is no reason why it should be otherwise, because the first win at Old Trafford occurred without Van Persie and Thierry Henry.
2) The defense has given up some big games. Man Utd's last season was by and large smooth sailing but there have been concerns. Both games against Arsenal were losses, shock wins by Portsmouth and West Ham, and the defense was schooled by AC Milan and Chelsea, in the Champions league semi-finals and the FA Cup final, respectively. Injury problems have smitten Gary Neville and Rio Ferdinand. Wes Brown and Gaby Heinze have not been the brightest sparks. Nemanja Vidic is the only defender who earned his spot. Evra is a speedy wing back but his tackling skills are a bit suspect. At this juncture I would take Man City's defense in a heartbeat. Micah Richards is the future of the English squad and Vedran Corluka is going to be a force to reckon with. Man Utd looked in a defensive meltdown against Inter in a friendly.
3) The Gaby Heinze factor: Remember how Sir Alex singled out the improved locker room atmosphere for the Premiership win, implicating Van Nistelrooy's departure as a key factor. Well, the Heinze factor basically sets that back. You have a vitiated atmosphere in Man Utd because of a disgruntled player who has been refused a transfer to one of its bitterest rivals. Yes, Gaby Heinze has been consigned to the reserves for daring to negotiate with Liverpool. He is going to chip away at the locker room camaraderie with his unhappiness.
4) The Carlos Tevez factor: Something tells me that this Carlos Tevez imbroglio that has already set Sir Alex's teeth on edge is going to come back. With a number of strikers at Sir Alex's disposal, each one a transfer target, investing so much on one striker, is going to have an unsettling effect on the rest of the attack. It just says that Sir Alex's trust goes so far. Such a contrast to Arsene Wenger's approach that has seen him repose faith in his present attack.