Genesis 4:2b-5 (The Living Bible)

[joannaingram84]

Abel became a shepherd, while Cain was a farmer. At harvest time Cain brought the Lord a gift of his farm produce, and Abel brought the fatty cuts of meat from his best lambs, and presented them to the Lord. And the Lord accepted Abel's offering, but not Cain's. This made Cain both dejected and very angry, and his face grew dark with fury.

[marciaenglish]

Why? 

[falcon9]

Why? 

Even though it was derived from much earlier pagan ritual sacrifice practices, it was often believed that blood sacrifices were 'stronger' than plant offerings to the deities, (although various pagan god/desses were offered either, depending upon the presummed aspects of the deital forms).  Apparently, the xtian derivative's preference leaned the more blood-thirsty variety.