TL;DR because most mammals can't even see green, stemming from a lack of advanced color vision due to our subterranean, twilight-dwelling lifestyle — whereas birds and reptiles have had the limelight for a long time. read more
The process by which mammals and reptiles get their color is different. Mammals use a pigment that absorbs and reflects specific colors. The primary skin pigment in mammals is melanin which has yellow, red, and brown forms. At high levels, it looks black. There doesn’t seem to be a green form of melanin. There are a few green mammals. read more
THERE ARE in fact green mammals: the two-toed sloth and three-toed sloth ( Choloepus and Bradypus ). However, these are not truly green, but have specially adapted grooves in the hairs of their fur to which cling a blue-green algae ( cyanophyta ). read more
So the small portion of reptiles that live among green leaves tend to be green, and the vast majority of reptiles that live on the ground, under leaf litter, on tree trunks, or in ponds, are brown! Only a small portion are brightly colored for predator warnings and/or social / sexual reasons. read more