Because (I quote from the ArtScroll Chumash's commentaries) "Abel feared
G-d's curse against the ground, he turned to caring for sheep and herds (Rashi
)". Although Man was still forbidden to eat meat [see 9:3 (Bereishis)], he
was allowed to use milk, butter, wool, and the skins of dead animals. Abel's
work consisted of shearing the sheep and milking the cows.
Like the Patriarchs, Moses, and David, Abel chose a profession that permitted
him to spend his time in solitude and contemplation of spiritual matters (HaK'