Why? Because they did not pursue it by faith, but as though it were by works. They stumbled over the stumbling stone,

just as it is written,
“Behold, I lay in Zion a stone of stumbling and a rock of offense,
And he who believes in Him will not be disappointed.”

and,
“A stone of stumbling and a rock of offense”;
for they stumble because they are disobedient to the word, and to this doom they were also appointed.