Jamieson, Fausset and Brown on Matthew 19:8

8. He saith unto them, Moses--as a civil lawgiver.

because of--or "having respect to."

the hardness of your hearts--looking to your low moral state, and your inability to endure the strictness of the original law.

suffered you to put away your wives--tolerated a relaxation of the strictness of the marriage bond--not as approving of it, but to prevent still greater evils.

But from the beginning it was not so--This is repeated, in order to impress upon His audience the temporary and purely civil character of this Mosaic relaxation.