Why didn't G-d accept Cain's offering, but accepted Abel's offering?

We must assume that Abel did what Adam had told him he should, as G-d had instructed, bringing the offering of the firstborn of his flock, an innocent lamb to die for his sins.
But Cain apparently didn't see the need to exchange his crops for a lamb, but instead presented the works of his own hands to G-d, and expected G-d to accept it anyway, though it was contrary to what he undoubtedly had been told G-d required him to do.
It's the same way we all are. We would rather try to earn our way to heaven with the works of our own hands, than seek forgiveness of our sins by pleading the innocent blood of the Lamb. We would rather do anything other than face the facts of our own sins, and our own inability to ever do anything about our sin nature.
It's works versus grace, trying to do for ourselves what only G-d can do for us, save us.