Because they believed not in God, and trusted not in his salvation:
ZEFANIABIBLE_BIBLE_BOOK_NAME_19 106:24
They despised the pleasant land and did not believe His promise.
ZEFANIABIBLE_BIBLE_BOOK_NAME_23 7:9
the head of Ephraim is Samaria, and the head of Samaria is the son of Remaliah. If you do not stand firm in your faith, then you will not stand at all.
ZEFANIABIBLE_BIBLE_BOOK_NAME_58 3:12
Watch out, brothers, so that there won't be in any of you an evil, unbelieving heart that departs from the living God.
ZEFANIABIBLE_BIBLE_BOOK_NAME_58 3:18
And to whom did He "swear that they would not enter His rest," if not those who disobeyed?
ZEFANIABIBLE_BIBLE_BOOK_NAME_58 3:19
So we see that they were unable to enter because of unbelief.
ZEFANIABIBLE_BIBLE_BOOK_NAME_58 11:6
Now without faith it is impossible to please God, for the one who draws near to Him must believe that He exists and rewards those who seek Him.
ZEFANIABIBLE_BIBLE_BOOK_NAME_62 5:10
(The one who believes in the Son of God has the testimony in himself. The one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given about His Son.)
ZEFANIABIBLE_BIBLE_BOOK_NAME_65 1:5
Now I want to remind you, though you know all these things: the Lord, having first of all saved a people out of Egypt, later destroyed those who did not believe;