Why did Jan van Eyck use the grisaille method?

I believe Van Eyck used the Grisaille method due to the mediums he was using at the time. The underpainting was done in Tempera, a medium that was commonly used in his day.

The glazing, however, was not commonly used in his day. The combination of the two steps: the underpainting and the glazing, created a very realistic effect. Van Eyck was a great innovator of developing a stable glazing medium using resins and nut oils, and is credited with being the first one of the first to use them as such.