According to The People History, fashion changes because the style is a fad, because it is dependent on the tastes of a particular group of people or because circumstances in a culture or economy change. The site also points out that most fashions never go out of style but simply change slightly to fit different trends.
Some fashion styles are considered fads because they are only popular for a season and are never worn again. The main difference between fads and other fashion styles is that fads are usually not able to change to keep up with trends. Thus, the success of fads are usually short lived.
Other fashion styles are dependent on particular groups of people. This is often seen when movie stars or music groups wear a specific style of clothing. This type of fashion is reliant on celebrities and their success. As celebrities change their tastes or new stars move into the limelight, the style of fashion changes with them.
Fashion may also change with world events. For example, during WWII, people were only allowed certain amounts of fabric with which to make clothes. As a result, fashion had to change in order to fit the limited resources of the time.