https://enduringword.com/bible-commentary/genesis-4/

c. The LORD respected Abel and his offering, but He did not respect Cain and his offering: Abel brought an offering of blood (the firstborn of his flock) and Cain brought an offering of vegetation (the fruit of the ground). Many assume that this was the difference between their offerings, but grain offerings were acceptable before God (as seen in Leviticus 2), though not as an atonement for sin.

i. “The word for offering, minchah, is used in its broadest sense, covering any type of gift man may bring… Neither of the two sacrifices is made specifically for sin. Nothing in the account points in this direction.” (Leupold)

ii. The writer to the Hebrews clearly explained why the offering of Abel was accepted and the offering of Cain was rejected: By faith Abel offered up a more excellent sacrifice than Cain (Hebrews 11:4). Cain’s offering was the effort of dead religion, while Abel’s offering was made in faith, in a desire to worship God in spirit and in truth.

d. Abel also brought of the firstborn of his flock and of their fat: This shows Abel’s offering was extra special. The fat of the animal was prized as its “luxury” and was to be given to God when the animal was sacrificed (Leviticus 3:16-17 and 7:23-25). The burning of fat in sacrifice before God is called a sweet aroma to the LORD (Leviticus 17:6).

i. The offering of Cain was no doubt more aesthetically pleasing; Abel’s would have been a bloody mess. But God was more concerned with faith in the heart than with artistic beauty.

ii. Here, it was one lamb for a man. Later, at the Passover, it will be one lamb for a family. Then, at the Day of Atonement, it was one lamb for the nation. Finally, with Jesus, there was one Lamb who took away the sin of the whole world (John 1:29).