Cain and Abel

Why was Abel's sacrifice accepted while Cain's was rejected? The traditional evangelical answer has been that Cain did not bring a blood offering while Abel did. However, if you look at the sacrifices it was not a sin offering but a thanks offering which did not require a blood sacrifice. Once again it was not the external but rather the internal, the condition of the heart. The new covenant is on the heart and does not depend upon the externals. While externals are important they are only important if they reflect the heart.