What happened to the robes in the Harry Potter movies? In the first couple of movies they wore their robes all the time, but by the end of the movie series there was little to no use of robes by the main cast, even thought they should have been wearing them the majority of the time.
Jason Baker's comment is correct.
Cuarón also made the characters feel a bit more contemporary via their wardrobe. In the previous films, the students had worn school uniforms and robes the majority of the time; in Azkaban, there was a shift. The young characters were now seen in normal, everyday clothes as a further reflection of their personalities and maturation. Too, they could individualize their uniforms a bit
Harry Potter: From Page to Screen: The Complete Filmmaking Journey by Bob McCabe - Page 104 - Prisoner of Azkaban analysis