I think the main reason people really like Cochins is their docile nature. Cochins look a little bigger than other breeds of bantams because they are heavily feathered. Silkies are another well sought breed of bantam by people who are into raising bantams. Silkies are very unique looking which really adds to their popularity. The feathers on a Silkie look more like hair than they do feathers.
Silkies are also very docile which really adds to their demand because they make good pets.
When I raised bantams my most popular seller was the White Crested Black Polish bantams. Everyone wanted these bantams and it was impossible to keep up with the demand for them. These bantams are pure black except for the puff of white feathers on their head. This white hat of feathers makes these bantams look like they are wearing a wig. There are countless other varieties that people enjoy raising but the ones I described are the most popular.