I think the reason they're not more popular is due to cost, complexity, and space concerns.
Cost - Any non-driving wheels will need another CV joint and bearing to hold the brake. Cooling ducts will probably be necessary, impacting the design of the whole vehicle.
Complexity - The above parts aren't free.
Space concerns - Two of the brakes are going to take space away from the engine compartment, which is usually cramped already.
The result does lower unsprung weight (improving grip), but also raises overall weight (reducing acceleration) and may hurt aerodynamics for the cooling ducts (reducing acceleration, top speed, and efficiency).
Everyone loves low unsprung weight but one has to ask if all the consequences of this particular technique are worth it.
-Ryan / Kardax