I answer a lot of questions on Quora. Many of those questions are about automotive things that may be of interest to CarNewsCafe readers. Like this one.
My Answer: Two reasons: the extra weight (mass) added with the components and power loss due to the added mechanics.
A two-wheel drive vehicle normally churns power from the engine to the transmission, which then sends power to the drive wheels. The mechanical losses are minimized by keeping the transitions of power from the engine as simple as possible. This is one reason that front-wheel drive is more efficient than rear-wheel drive, as the tiny percentage of power lost to the drive shaft and differential to run to the rear wheels is not present on the FWD car.
Adding a differential to create the split required to send power to both the front and rear wheels results in power loss due to the added mechanics (differential, extra drive shaft, rear differential hub). Even when that center differential disconnects to improve fuel economy when AWD is not required, those extra components are adding weight and mechanical drag to the car that reduce efficiency.
In four-wheel drive vehicles, the drag is even worse as the center differential is replaced with a transfer case (second gearbox) that adds even more complexity and weight.
The simpler the mechanics involved, the less power loss (in weight and mechanics) there is and the less work the engine must do to move the vehicle.