Toulouse-Lautrec  - Dancer Seated

From the middle of the Nineteenth Century, scenes of the circus, the theater and the ballet became popular. From the Renaissance onward, these had hardly been the subject of serious art.

What brought about the change? It was partly the result of a change in the ideal of feminine beauty. Ideals of feminine beauty have an even greater influence on iconography than general cultural attitudes have on the ideal of feminine beauty. And The Woman is such an important icon in Western Art that as goes The Woman, so go the visual arts.
Toward the middle of the Nineteenth Century, a type of lithe young woman came into fashion who for physical reasons was often to be found as an acrobat or a dancer.

The Realist tendency in all of the arts also helped. Realism gave legitimacy to the portayal of the not-quite-respectable world of acrobats and dancers.
Chagall  - The Acrobat

The Realist movement also permitted artists to portray their own not-quite-respectable world, a world which obviously held great interest for them, almost for the first time.

The blurring of the border between the serious arts and advertising also helped. Theaters, ballet troupes, and circuses obviously need advertising. When we think of how useful beautiful women are in advertisements, we've come full circle.

Spaces, not commas, please!