If You Will Not Redeem Him

I tend to find that seemingly archaic and useless laws from the Old Covenant tend to have great depth of meaning if we will take the time to think about them and appreciate them. One such law was that the firstborn offspring of any livestock had to be either sacrificed to God or paid for by a sacrifice:

Exodus 34:19-20
All that open the womb are Mine, and every male firstborn among your livestock, whether ox or sheep.  But the firstborn of a donkey you shall redeem with a lamb. And if you will not redeem him, then you shall break his neck. All the firstborn of your sons you shall redeem. And none shall appear before Me empty-handed.

Why did the donkey have to be redeemed instead of sacrificed like the ox or sheep?