The CAFC's rulings spell it out pretty plainly. There is sufficient original creative work in the choices underpinning the design of the Java APIs that they are eligible for copyright. There are no external forces compelling the Java APIs to conform to the exact shape they now hold, and the same systems and processes could be expressed in other ways. Therefore, the "system or method" exemption to copyrightability does not hold for the APIs.
By copying Java's APIs in a non-interoperable way, Google has diluted the Java ecosystem and sown confusion amongst users of the language, thus diminished the value of the IP held by Oracle, and Oracle is entitled to damages.
The CAFC's ruling was only clear because of the amazing amount of 9th circuit precedent they left out, going off and making their own law for some reason.
A number of industry professionals have also made this argument. See Eugene Spafford's amicus brief for an example: http://www.supremecourt.gov/DocketPDF/18/18-956/133474/20200...
By copying Java's APIs in a non-interoperable way, Google has diluted the Java ecosystem and sown confusion amongst users of the language, thus diminished the value of the IP held by Oracle, and Oracle is entitled to damages.