I Think --Faith is the issue -----
Faith in God is a requirement for God's approval -----Able showed he had Faith in God ----- He did not value his Flock more than God ---as Able picked the best from His Flock to present to God -----
Genesis 4:4 NIV
4 And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering,
I say -----Cain on the other hand showed he did not have Genuine Faith in God -----he actually showed no respect in what he offered -----and he responded badly when God refused hid offering -----so he showed no Faith and no respect for God ------
Genesis 4:5 AMP
5 but for Cain and his offering
He had no respect. So Cain became extremely angry (indignant), and [
a]he looked annoyed
and hostile.
Scripture clearly says ---without Faith we cannot please God ------