I'd lean toward a Scriptural reason, described in Romans 11. (And, yes, I do recommend reading the whole chapter!) There is a partial hardening until the fullness of the Gentiles comes in:

Romans 11:24 - 27

For if thou were cut out of that which is by nature a wild olive tree, and were grafted contrary to nature into a good olive tree; how much more shall these, which are the natural branches, be grafted into their own olive tree?

¶ For I would not, brethren, have you ignorant of this mystery, lest ye be wise in your own conceit, that a hardening in part has befallen Israel, until the fulness of the Gentiles has come in; Rom 11:26

and so all Israel shall be saved: even as it is written, There shall come out of Zion the Deliverer; He shall turn away ungodliness from Jacob: Rom 11:27

And this is my covenant unto them, When I shall take away their sins.

I'd lean toward a Scriptural reason, described in Romans 11. There is a partial hardening until the fullness of the Gentiles comes in:

Romans 11:24 - 27

For if thou were cut out of that which is by nature a wild olive tree, and were grafted contrary to nature into a good olive tree; how much more shall these, which are the natural branches, be grafted into their own olive tree?

¶ For I would not, brethren, have you ignorant of this mystery, lest ye be wise in your own conceit, that a hardening in part has befallen Israel, until the fulness of the Gentiles has come in; Rom 11:26

and so all Israel shall be saved: even as it is written, There shall come out of Zion the Deliverer; He shall turn away ungodliness from Jacob: Rom 11:27

And this is my covenant unto them, When I shall take away their sins.