Glare is caused from light bouncing off a surface directly into the camera lens. Most competing chromakey fabric, paper, or paints are extremely flat and highly susceptible to this glare. Even woven fabrics that say they are a "matte" or "non-glare" finish do not compare to the unique design on foam-backed chroma-key greenscreen and bluescreen  fabric.
(See image 1A to the left - Click to Enlarge)

EEFX.COM's Foam-Back Fabric:

The top layer of Foam-Backed chromakey fabric is composed of millions of tiny loops that break up the light rays by absorbing and scattering them. This allows you to easily light the backdrop evenly! (See image 2A to the left - Click to Enlarge)