When you read this stuff regularly enough you just kinda go, “oh ok, that again.” But I’ve never really read it explained. But in The Philosophy of Hebrew Scripture by Yoram Hazony I read an explanation of the distinction in terms of the ethic represented by the two characters. Here it is in full:
The life of the farmer. Cain has piously accepted the curse on the soil, and God’s having sent Adam to work the soil, as unchallengeable. His response is to submit, as his father did before him. And within the framework of this submission, he initiates ways of giving up what little he has as an offer of thanksgiving. In the eyes of the biblical author, Cain represents the life of the farmer, the life of pious submission, obeying in gratitude the custom that has been handed down, which alone provides bread so that man may live.
The life of the shepherd. Abel takes the curse on the soil as a fac, but not as one that possesses any intrinsic merit, so that it should command his allegiance. The fact that God has decreed it, and that his father has submitted to it, does not make it good. His response is the opposite of submittion: He resists with ingenuity and daring, risking the anger of man and God to secure the improvement for himself and for his children. Abel represents the life of the shepherd, which is a life of dissent and initiatve, whose aim is to find the good life for man, which is presumed to be God’s true will. (108)
The author goes on to observe that while God did not command shepherding, God did make man to be good. So when Cain was downcast, God told him “If you do well, won’t you be lifted up?” Meaning, why not work to improve your lot in life as you can? (108)
I find that argument to be convincing in light of some details the author does not include in his argument: God made man “very good” and commanded man to subdue the earth. So it doesn’t seem like God wanted man to submit directly to the curse. But to continue the mission from Genesis 1. The curse was not an end to the blessing at the creation of humanity, but merely a handicap.