It is the inner principle of sin. Covetousness is the defect in our internal power-steering mechanism that pulls us toward the wrong. A practical substitute for the word covet might be greed, provided we understand this can mean more than simply greed for money. We can be greedy for power, sex, approval, authority, praise, status and so forth. Another good reason for this commandment being last is because it's such a universal sin.
Covetousness is the last enemy of both the sinner and the saint.
Almost every language and culture have proverbs about covetousness. Covetousness affects every level of human life. We find it in the church, ourselves and our communities. Christ taught sacrifice and selflessness, yet we see little of that in our lives. Even godly ministers have fallen prey to covetousness by desiring prestige and authority in the church.