Why did Mondrian only use primary colors?

Mondrian use of simple blocks of colour is deceptive in its simplicity. His non-representational form which he termed Neo-Plasticism consisted of a grid of vertical and horizontal black lines and the use of the three primary colors in the created spaces. On close inspection the black of the lines is as much part of the colour scheme as the blocks. Neither blocks or lines are simple forms as they have slight variations in thickness and alignment.