Do I need to point out the elephant in the room?
Evil is (typically) easy; Good is (typically) hard. Giving into fear, jealousy, hatred, greed, prejudice, et cetera, requires less effort than making consistent efforts to check these darker thoughts and then rise above them. The fact that a priest of an Evil god receives potent spells/various divine gifts by channeling their base emotions through worship is incentive enough to take a chance.
