Romans 4:16

Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham’s offspring—not only to those who are of the law but also to those who have the faith of Abraham. He is the father of us all.

(v16, NIV)

It doesn't make sense for the crediting of righteousness to come by the law. Following Abraham's example, Paul points to him and his promise, showing the way in which Abraham lived and how he was credited righteousness. If you recall, Paul's previous point was about the nature of works and how a worker is owed his wages. However, one who is graced with a gift is not owed in the same way a worker is owed his wages. If righteousness was based upon one's keeping of the law, it would be dependent on them and their performance. God's promise would be conditional and based upon the deeds of man instead of the faithfulness of God. Being by faith allows it to be by his grace and guaranteed according to the promise given to Abraham.