Two theories about why God accepted Abel’s offering of an animal but rejected Cain’s offering of crops. (1) Abel brought a premium offering; Cain brought the discount offering. (2) Abel had the right motive; Cain didn’t. A Visual Walk Through Genesis, page 44.
Also worth noting; cains offering came by the works of his hand and Abels by spotless sacrifice was an animal created by God. And the bible says without the shedding of blood there is no remission of sins.
Stephen M. Miller