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sbbohr-
Because the cost/benefit analysis is marginal when the faff of wiring in a rear dynamo light is taken into account, particularly if you're going to do it neatly and not have zip ties and electrical tape everywhere.
And moving a battery-powered light is far easier if it turns out your saddle-bag/top bag/panniers obscure the rear light as you don't have to worry about cables.
Basically, cos wiring.
Brommers
The power consumption for a rear light is miniscule compared to a front light, partially because its only purpose is to be seen rather than illuminate the road, and partially because red LEDs are more efficient anyway, as they don't need the floursecent coating that allows white LEDs to be white. A good battery powered rear light will last months on a single set of batteries. A good front light won't. Having a dynamo powered front and battery rears makes sense to me.