Why is the fear of God not mentioned in the New Testament like in the Old Testament?

It is! If you read the Bible carefully, it is there.

In fact, if you take a concordance and look for the words "fear of God," let me give you one example—2 Corinthians 5:11, "Knowing the fear of the Lord, we persuade men." So, it's through the fear of God that we seek to please the Lord, because of the judgement seat of Christ we stand before (verse 10). Also, 2 Corinthians 7:1, "We perfect holiness in the fear of God."

It's a very sad thing that many believers do not have the fear of God. It says even about Jesus that he overcame sin because of his fear of the Father. In Hebrews 5:7, he prayed with loud crying and tears to avoid sinning—that is, to be saved from spiritual death, be saved from sin. He prayed and he was saved, and he was heard because of his godly fear. So, it's mentioned in the Bible a number of times, even in the new covenant, and it's a sad thing that many believers don't have it.

In fact, we must know the fear of God before we know the grace of God. The early disciples, for 30 years they'd learned the fear of God, then they learned about the grace of God. The unfortunate thing today is many believers know about the grace of God, but they don't have a clue about the fear of God, and that's why they're so defeated and a disgrace to Christ.

We need to learn the fear of God is the beginning of wisdom even today. Psalm 25:14 says, "the secret of the Lord is with those who fear him." God does not reveal his secrets to those who don't fear him.