I think its because they can't, It would either cost way too much to produce or they dont have the facility to produce it, it would take a lot (or would cost a lot) to mass produce cars, even then if they only made hundreds it would then mean each one would cost way too much.
The only way it could be done would be if subaru made the car, but then a) it wouldn't be a pro drive car b) the impreza would take a bit of a hit. Then factor in the major thing about the top spec 4wd versions, mainly bought by enthusiasts and they have to produce X ammount to enter into the rallying, which they could only really do but under subaru.
So if it were to be produced as a car, the only way I could see it happening would be through homogenation via a rally car, but then whats the point since subaru use pro drive anyway, the costs doing it any other way wouldn't make economic sense, if it did they would screw subaru and become a major car company probably overtaking subaru in the 4wd rally area.
If it did make economic sense to make (and could make a profit) there would be no way anyone would turn that down. The technical side of it is there but I think its a bit like the Bugatti, a show off toy of their performance not intended for mass production ( although it looks like the Bugatti is going that way!) and it has worked, it has put pro drive on the map for the masses