Bonnet struts are considered a hazard in a front end impact and manufacturers lose NCAP points for them, hence only one if they can get away with it, or none if they're close to the limit points-wise.
Its also a lot lighter so one is really all thats needed.
The danger at the rear is that uneven tortion might induce twisting, and in a structure with a dirty great window glued to it that's not a desirable state of affairs.