"English-language arts and math were the first subjects chosen for the common core state standards because these two subjects are skills, upon which students build skill sets in other subject areas. They are also the subjects most frequently assessed for accountability purposes.
Of course, other subject areas are critical to young people's education and their success in college and careers. Once the English-language arts and math standards are developed, CCSSO and NGA Center, on behalf of the states, plan to develop a common core of standards in science and potentially additional subject areas." Source: corestandards.org/assets/CoreFAQ.pdf