I don't buy the standard answer on this subject.
Seems worthy of discussion.
Let's explore some theories. Thanks.
The standard answer is that Abel's offering was a blood sacrifice.
I don't think that was the reason that Cain's offering was rejected.
They both brought something from the work of their hands.
Was Cain condemned for working the soil rather than keeping a flock?
Genesis 3:21 Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.
(NOTE: Shortly after Adam & Eve sin. God made coats of skins to clothe/cover their sin. I believe we see here the first instance of the blood of the innocent paying for the sin of the guilty)
Hebrews 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh
(NOTE: Abel brought his offering via FAITH. Obviously Cain brought his without faith. God views our own self righteous works as filthy rags - Isa 64:6)
Hebrews 11:6 without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him
(NOTE: Abel's sacrifice was brought in FAITH)
Genesis 4:
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
(NOTE: Abel is a shepherd & Cain a farmer)
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
(NOTE: Cain's offering is bloodless & 100% self works. 1 Jn 3:12 tells Cain's own works were evil & Abel's was righteous)
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
(NOTE: Lev 3:16 the fattest, best of your flock, defect & blemish free, not lame or sick. God is to be, given, served with, the best we have. Abel's offering was, perfect & without spot. A shadow of Christ, God's sacrificial Lamb ,to come.
In later time's the fat was claimed by the Lord as his own (Lev 7
Another key here is firstling. (Lev 23:10-11) the Temple Priests were to bring a firstfruits (Christ rises on the feast of firstfruits) offering & wave it up before the Lord. This firstfruits wave then sanctified the whole field of crops. 1 Cor 15:20-22 Paul refers to Christ as the firstfruit of the resurrection. Whereby everyone, the whole field/human race that ever dies with be resurrected. Some to eternal life with God & other eternal separation from God)
Finally, I believe this passage of the LORDS respect for Abel's offer, implies: That with divine approval, God shot fire from heaven onto Abel's offering. Also see Lev 9:23-24, 1 Kings 18:22-38, 1 Chron 21:26, 2 Chron 7:1)
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
(NOTE: Conclusion, the Lord had no respect for Cain's faithless, self righteous offering) MOO!