I believe it's because divorce is indicative of our, as the Bride of Christ, putting material things and this world or others before God. God tells us that He is a jealous God.
Ex 20:5 you shall not bow down to them nor serve them. For I, the Lord your God, am a jealous God, visiting the iniquity of the fathers on the children to the third and fourth generations of those who hate Me,
Ex 34:14 (for you shall worship no other god, for the Lord, whose name is Jealous, is a jealous God),
De 4:24 For the Lord your God is a consuming fire, a jealous God.
De 5:9 you shall not bow down to them nor serve them. For I, the Lord your God, am a jealous God, visiting the iniquity of the fathers upon the children to the third and fourth generations of those who hate Me,
De 6:15 (for the Lord your God is a jealous God among you), lest the anger of the Lord your God be aroused against you and destroy you from the face of the earth.
Jos 24:19 But Joshua said to the people, "You cannot serve the Lord, for He is a holy God. He is a jealous God; He will not forgive your transgressions nor your sins.
Eze 39:25 "Therefore thus says the Lord God: 'Now I will bring back the captives of Jacob, and have mercy on the whole house of Israel; and I will be jealous for My holy name--
Na 1:2 God is jealous, and the Lord avenges; The Lord avenges and is furious. The Lord will take vengeance on His adversaries, And He reserves wrath for His enemies;