I mean, they both brought stuff. Maybe Cain didn't have a particularly good harvest that year? All the bible says is:
Now Abel kept flocks, and Cain worked the soil. ~Genesis 4:2
That's hardly enough information to determine the actual reason why Cain only brought "some of the fruits of the soil".
Did you think that maybe Cain, being the elder child, wanted to keep some of the fruits for his parents too?
• The Q'uran has a similar story of Adam & Eve's sons killing each other, but no names are mentioned.
• A few scholars suggest the periscope may have been based on a Sumerian story representing the conflict between nomadic shepherds and settled farmers.