It's not cost,as they are more expensive than regular lights, although they do last a great deal longer than conventional lights. It's because of the plethora of electronic devices showing up in our vehicles. Navigation systems, tracton control, stability control, ABS brakes, all of this plus the more sophisticated computers runnig cars are taxing the electrical systems of modern vehicles. Today vehicles operate on 12 volt electrical systems, which are rated at 800 to 1500 watts, but it's estimated that future cars will require 3,00 to 7,000 watts, something a 12 V system just can't handle.
The next step up will be 42 Volts, which is the equivalent of three 12V (12*3=36) batteries, since todays batteries are actually 14V (14*3=42), even though it's stated as 12V. Bigger batteries are on he way, and during the transition you might see cars with two batteries, one to provide power to the systems that require 42V (power steering, traction control,brake, and engine cooling systems) and a regular 14V battery to provide the spark to power low-load systems (lights, power door locks, radios).
Imagins trying to jump-start a car with two batteries!