1.) Wouldn't be a guaranteed starter.
While he has been dropped for the occasional game, the Champions League quarter final second leg against Real Madrid springs to mind, Rooney has largely been guaranteed a starting spot in the side. Out of the four strikers at United, Rooney is the more accomplished in the centre forward position, while Robin Van Persie, Javier Hernandez and Danny Welbeck are more accustomed to playing higher up the pitch in a poachers role. This means rotation in the strike force happens but to the other three strikers, the only player likely to replace Rooney is Shinji Kagawa who can also play "in the hole". A switch to Barcelona would seem him drop down the pecking order and be placed behind the likes of Lionel Messi and Cesc Fàbregas.
2.) There is no natural position for him.
Rooney's position is often described as playing "in the hole" meaning the gap between the midfield and the front strikers. This position is not used by Barcelona as they favour a false 9 with Messi or a false 10 with Fàbregas. Rooney has also occasionally played as a midfielder for Manchester United but even then he was used as a box-to-box midfielder and remained very strict in his position. Which the likes of Xavi, Iniesta and Busquets do not as they favour a more fluid approach which allows the players to swap positions fast and effectively.
3.) Wouldn't be an icon of the team.
For the past eight years, Rooney has been the face of United's advertising campaign. Appearing in practically all promotion, including a dodgy wine advert , and in doing so has become the superstar at United. A switch to Barcelona would not only see him slip behind Messi but also Xavi, Iniesta, Fàbregas and practically anyone who is a graduate of La Masia. While many argue this would be good for a character such as Rooney, his ego may not be able stomach it.
4.) His style is outdated.
Rooney is often described here in England as the typical English player, strong, likes a physical game and prefers power over precision. This style has become outdated in recent years especially with the rise of Barcelona's tiki-taka style but aspects of Rooney are not. For example Cristiano Ronaldo is a strong player but has coupled it with speed and has become one of the best players in the world, even his strike partner Robin Van Persie uses power but also has immense precision when shooting at goal. Rooney, though, seems unable to develop his skills and the world of football appears to be moving on without him.
5.) His wages would be cut.
Wayne Rooney is on a reported £300,000 (approx €351,906) a week salary at United. While Barcelona's highest earner, Lionel Messi, is on a reported £275,000 (approx €322,580) a week salary. Meaning Barcelona would be unlikely to offer anything anywhere near Rooney's current contract, most likely they would offer something in the region of £150,000 - £200,000 a week for Rooney meaning the United forward stands to lose around £150,000 a week. Which could be a huge deciding factor should Rooney's transfer request be granted.