Because in their spring 2011 shoe collection, Yves Saint Laurent included a pair of red suede pumps with matching red soles. Apparently Louboutin thinks he is the only designer entitled to have red soles on his shoes. Even though YSL‘s collection also included purple pumps with purple soles, navy pumps with navy soles, and green pumps with green soles.
Louboutin’s argument is that the red soles may cause confusion, mistake and deception for the public about the origin of the shoes, since red soles are a popular mark of Louboutin shoes.
(the pumps pictured are from a past season collection for YSL)