John Gill Commentary Mark 10:5

"But Jesus said unto them, For your hardness of heart he wrote you this commandment." — Mark 10:5 (ASV)

And Jesus answered and said to them
With respect to this command, or sufferance of Moses, which they urged:

for the hardness of your heart he wrote you this precept
it was, not because it was right in its own nature, or according to the original will of God; but, because the Jews were such cruel, and hard-hearted men, that if this had not been permitted, some of them, that had wives not so agreeable to them, would have used them in a very inhuman manner, if not murdered them; and therefore to prevent further and greater mischief, Moses indulged them with such a precept; (See Gill on Matthew 19:8).