Remember that light is synthesized by combining the primaries; the spectrum is defined by their convex hull. You can expand the hull and ergo gamut by adding primaries.

The RGB setup we have strikes a balance between cost and visual quality. If the cost of adding primaries goes down you can add more to increase the quality. One issue is that the signals often assume RGB (channels), so the hardware manufacturer would have to adapt the RGB signal to their multi-primary hardware.