The short, pretty, frilly tutus worn by ballerinas today come from the changes in dance and dancewear that have happened over time!
Several hundred years ago when ballet was first becoming popular, the types of outfits worn by ballerinas today (like leotards and tights) didn’t exist yet. Girls and women who danced ballet wore long dresses that covered their legs, because it was traditional at that time for women to wear this style of clothing.
In the 1700-1800s, dancers began to shorten the length of their dresses in order to better show off all of the special movements they were doing with their legs. In order to keep themselves covered as they moved, the dancers had to wear layers with special shorts-like garments (petticoats) under their dresses.
Over time, the dresses became shorter and shorter as styles changed. Fashions like leotards and tights became preferable to dresses. The types of tutus worn today are what remains of the old tradition of wearing layers and petticoats!