Well, suppose I want a book about dragons. In 3E I could get Draconomicon and be happy. Now I'll have to either pick a type of dragon, or pay twice more. Even if the books have more content together than the 3E counterpart, agree with me that it's a much less attractive option for the budget-minded.
And please don't compare it to the demon/devil 3E books. Grouping them into a single "fiends" category is not like grouping chromatic and metallic dragons together. There's plenty to explore and expand on demons and devils separately, but how many differences could chromatic and metallic dragons, as subtypes, possibly have? (Unless WOTC are planning surprises there, of course...)