God respected Abel, but not Cain.

Abel offering unto God a more excellent sacrifice than Cain. When we read Genesis 4, we read that God accepted Abel's person and then his offering. In contrast, God rejected Cain and also his offering. So don't focus only on their offerings and think to yourself that Abel had correct offering and Cain didn't. Although I do believe Abel brought "a more excellent offering," I would tell you that if Cain would have gone back and brought the identical offering that Abel brought, God would have rejected Cain, again! And that is because he brought his offering with a wicked heart.