That probably wouldn't have worked with what they were going for. The reason Ratchet's movie designed remained relatively unchanged was because he's açready quite detailed in the games, however, no matter how you look at it, you can't tell me Sly's model in TiT is detailed since it's obviously going for a stylised cartoon art style. Sly in the movie was going in a more… realistic route, and thus his design from TiT wouldn't work. Now for my second reason. Ratchet is a lombax, a made up and alien species. However, most of the characters in Sly are real life animals, Sly being a racoon, so in order for this more detailed approach to work, they'd need to give Sly some more "racoon-y" features in order to not make him look odd, hence the whiskers, the lips, the ears, the teeth… (and probably the pants )