Why was a raven chosen to feed Elijah?

Let's consider a few points

1. Elijah did not eat the ravens and there is no indication that the food he ate from the ravens were unclean.

2. If you consider God's revelation to Peter in Acts 10:15 when Peter refused to eat unclean animals, God expressly told him "What God hath cleansed, that call not thou common." We can say Elijah's story was a hint to God's plan for humanity in the new covenant.

3. In other passages where Raven was mentioned in the Bible, Psalm 147:9, Job 38:41 and Luke 12:24, they were used as an example of God's gracious provision for all his creatures.