Mainly because they are not in demand
Asian (Grass) carp are an invasive species (controlled), Common carp are an introduced species (for food, pre 1900, as are brook trout, brown trout, and kokanee) and seem to be regarded as less problematic than asian carp. The English get nuts about fishing for carp - their carp are a lot smarter (better educated) than our carp, and a challenge to catch (carp have 6 or 7 senses including great hearing, and
are pretty smart fish). I hear they are a hoot on a fly rod, if you can get a bite.