Why did God favor Abel’s gift but not Cain’s?

God instructed us to till the ground in Genesis 2:15, so it wasn’t the case that Cain’s offering of fruit was rejected on the basis that God did not want him to be a tiller of the soil. Rather, I believe it had to do with his motive, in contrast to Abel’s motive, whose offering was favored. Hebrews 11:4 tells us that Abel’s gift was favored because it was offered from faith. Abel offered to God “the firstborn and fattest” of his flock. In other words, he offered the very best of what he had. It doesn’t specify anything about Cain’s gift other than it being “some of the fruits of the soil”. The lack of qualifier suggests that Cain may not have offered his best to God. Some translations obscure this distinction when trying to make sense of the Hebrew phrase:  וּמֵֽחֶלְבֵהֶ֑ן צֹאנ֖וֹ מִבְּכֹר֥וֹת, roughly translated as “the firstborn of the fat of his flock,” which only appears in the scripture this one time. The idea is that God wants us to always give our best to him. Anything short is wrong and unacceptable. God says to Cain in Genesis 4:7, “If you do what is right, will you not be accepted?” Instead of humbly repenting and wholeheartedly offering his best, Cain became angry and killed his brother, which made matters worse for him.