Abrahamism is anti-Sumerian in its doctrine. Shepherding and pastoralism are seen as closer to God. Cain represents the Sumerian way of life, which is why he goes on to found a city, whereas Abel represents the Israelite way of life.
It is important to note that shepherds were usually mostly lactovegetarians, whereas Sumerians probably had lots of farmed, unpastorable meat like chicken and pork, which were seen as unclean due to this association. This noble idea of the shepherd juxtaposed against the jealous mercantile civilization is also visible in Hinduism. It is part of why vegetarianism is as big as it is and why the cow, and especially the cow’s milk, is so revered.
Abel’s sacrifice is accepted because a shepherd in those days was very close to their animals. Killing an animal because there was no milk and the only other option was to starve was seen as noble. Shepherding was not seen as a type of farming, but a way of life where every day you are protecting this flock of animals who depend on you and love you from wild predators. In return they give their milk, and in desperate times, their flesh. The act of sacrificing an animal was seen as deeply emotional and a true sacrifice.
God was not impressed by simple grain, as it was not a true sacrifice.