And why? Because they sought it not by faith, but thought to gain it by works. They stumbled at the stone of stumbling;

even as it is written. Behold, I lay in Zion a stone of stumbling and a rock of offense; but he that believes on Him shall not be put to shame.

and, A stone of stumbling, and a rock of offense. They stumble over it because they are disobeying God's word, and to this they were also appointed.