They nailed it on the dot... M5 sings to everyone' hearts because it's a race car in a saloon body. All the competition is around the same weight but the M5 just does it better.
The M6 on the otherhand is fighting in a market where its competition is smaller, sexier, sleeker, and most importantly, lighter. Sports cars are supposed to be small, agile, and sexy... the M6 is just a huge GT. There is no purpose to make an M6, they need to come out with a true sports car body + platform and drop the M5 drivetrain in. Obviously higher R&D and probably will not make a good business case. But hey... Audi got its R8 and it's beyond be why the hell anyone would pay effin $120+ for a Carerra S these days nonetheless the price on the turbo/gt3 when it comes out.