Why did Piet Mondrian reject Theo van Doesburg’s use of diagonal lines?

Piet Mondrian rejected Theo van Doesburg’s use of diagonal lines because they conflicted with his fundamental principles of Neoplasticism.

Mondrian sought to express a "true vision of reality" and universal harmony by reducing painting to its most basic, objective elements: primary colors and the intersection of vertical and horizontal lines. He believed that this strict geometric language was the most effective way to convey his spiritual and philosophical beliefs, free from the artist's personal temperament or the representation of momentary visual perceptions.12

When van Doesburg introduced diagonal lines into his compositions around 1924—a style he termed "elementarism"—he intended to create a more dynamic effect. Mondrian, however, viewed this as a departure from the austere, balanced order he had established for the movement. Because Mondrian felt so strongly that these diagonals undermined the clarity and equilibrium of his artistic theories, he ultimately rejected the De Stijl movement.3