The A15 cores are new. the time that sammy has been developing the SGS3, the tech probably wasnt available in order to launch the phone now.
Im pretty excited to pick up one of these bad boys June 20th (hopefully).. Im not due for an upgrade but I think I might threaten to jump ship if they dont upgrade me as ive heard a lot of leeway going on for those upgrading 3g>4g phones..