Why Cubist artists were rebels?

Rebels of a New Generation

Cubist artists rebelled against the conventional representation of forms by turning them into fractured, geometrical shapes. That was yet another step towards complete abstraction seen in the Abstract styles and Futurism.

They also challenged the very idea of what a painting is. Conventionally, you’d expect only, well, paint! They introduced new materials transforming the traditional 2-dimensional painting into a 3-dimensional collage. That which was revolutionary a hundred years ago is now ironically a standard part of art classes in schools, i.e. creating collages.