They're typically expensive, heavy, slowing, and impose a great armor check penalty.
I can get a chain shirt (4AC, 100G, 25lb), dastana (1AC, 25G, 5lb, Arms and Equipment Guide 15), and a chahar-aina (1AC, 75G, 10lb, Oriental Adventures 76) for far less than a suit of full plate (8AC, 1500G, 50lb). There's only a 2 AC difference, but with the light armor set I'm ahead by 4ACP and 15lb.
I can also Tumble in light armor as any race.
I'm not seeing the point of other armors for most characters.