Am just putting out there that maybe abel was smarter than Cain by keeping sheep, while Cain chose to till the ground that was cursed. But it could have been adam and eve made that choice for them or told them to do those things.
But if Cain was smart he would have realised the ground was cursed with thorns and thistles and nothing good could come of it. After all they were banned from the garden of eden thanks to their parents and couldnt access the tree of life.
God said to Cain if thou doest well shall, thou not be accepted? What is it about keeping sheep that was acceptable...? Note that Jesus talks about being the good shepherd, telling Peter to feed my sheep, feed my lambs. Note too that David was a shepherd boy, proecting his lambs.
Note that jesus was declared the Lamb of God.
Was it anything to do with SHEEP? no other animal was offered...and they had all kinds of animals then too.