Either,
1. Because the one was a clean beast, the other unclean; whereby God would teach men to avoid polluting themselves by the touch of unclean persons or things, 2 Corinthians 6:14. Or,
2. Because of their unequal strength, whereby the weaker, the ass, would be oppressed and overwrought. Or,
3. For mystical reasons, of which see on Deuteronomy 22:9 Leviticus 19:19.