WSIL -- Despite Halloween being a holiday with a long history, dressing up only became popular in the 20th century. In fact, store bought costumes weren't available before the 1930s.
Hollywood had a big influence on the costumes being made. Actors like Lon Chaney, who played Wolf-Man, and Bela Lugosi, who was Count Dracula, inspired a lot of trends in the 40s and 50s. As trick or treating became popular, so did the tradition of kids dressing in costume.
Clowns, pirates, fairies and super heroes soon became the trend. In the 70s and 80s costumes were made of plastic and included a mask that made it nearly impossible to see through.
Now costumes are made from cloth and are better fitting and safer in general.