Why would God permit divorce because of Israel's "hardness of heart"? When questioned about divorce, Jesus refers to the Mosaic law and explains why God allowed divorce certificates in the first place.


  Mark 10:2-9 ESV And Pharisees came up and in order to test him asked, “Is it lawful for a man to divorce his wife?” He answered them, “What did Moses command you?” They said, “Moses allowed a man to write a certificate of divorce and to send her away.” And Jesus said to them, “Because of your hardness of heart he wrote you this commandment. But from the beginning of creation, ‘God made them male and female.’ ‘Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.’ So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”


The certificate of divorce reference is, I'm assuming, from Deuteronomy 24.

This seems like an unusual thing for God to do: to explicitly allow something because of Israel's stubbornness. Certainly God hates divorce as much as other relational sins but he didn't tell Moses to give allowances for other things like fighting or idolatry.

Why would God permit a certificate of divorce because of Israel's stubbornness and not allow certificates for other wrongs? What makes divorce worthy of this? Is there additional context or statements regarding this in the bible?
The translation is not accurate. Original Greek says "is it lawful for a man to dismiss his wife?" Not divorce. The Pharisees would never ask "is it lawful for a man to divorce his wife?" as it is very clearly stated in Torah that it is (Deu. 24). Rather the question is about a man leaving his wife through separation alone. They ask Jesus why did Moses permit both divorce AND separation. Moses permitted separation, but Jesus reminded them that in separation you continue married and anything you do with another partner is considered adultery. 
The greek word in question is apoluo ap-ol-oo'-o from Strong's G575 and G3089 meaning; to free fully, i.e. (literally) relieve, release, dismiss (reflexively, depart), or (figuratively) let die, pardon or (specially) divorce. It is used many times throughout the New Testament such as when Pilate wanted to APOLUO Jesus (Luk. 23:20), meaning let him go, not divorce him. Or Acts 28:18 when Paul is talking about being let go by the Romans, not divorced by them. Apoluo is referring to being separated from, while bill of divorce refers to a legal divorce of marriage. –