Why did Edgar Degas Paint So Many Ballerinas?

I am a presenter for our Elementary School's Art Smarts Program. Just last year we presented Edgar Degas. Edgar Degas began painting ballerinas because he loved ballet, and because he loved the artistry and power of movement. For similar reasons he also painted many beautiful pictures of racehorses. Later, he continued to paint and draw ballerinas for two additional reasons: first, the ballerina pictures sold so well that they were making him wealthy, and, second, as he was losing his sight, it was easier for him to use pastels to draw a topic with which he had so much familiarity.