Because the letter was addressing the situation that the Thessalonians and other churches were under prior to the destruction of Jerusalem - this is from another thread:
The problem with the late "rapture" theory is that is fails to take note of the full context of the "occasional" nature of the letters to the Thessalonians.
2 Th 1:4 therefore, we ourselves speak proudly of you among the churches of God for your perseverance and faith in the midst of all your persecutions and afflictions which you endure.
2 Th 1:5 This is a plain indication of God’s righteous judgment so that you will be considered worthy of the kingdom of God, for which indeed you are suffering.
2 Th 1:6 For after all it is only just for God to repay with affliction those who afflict you,
2 Th 1:7 and to give relief to you who are afflicted and to us as well when the Lord Jesus will be revealed from heaven with His mighty angels in flaming fire,
Note that in the above Paul is promising relief and repayment to those that are afflicting THEM, and it was the Jews and their "henchman" that were doing the affliction. See his comments from his first letter to the Thessalonians:
1 Th 2:14 For you, brethren, became imitators of the churches of God in Christ Jesus that are in Judea, for you also endured the same sufferings at the hands of your own countrymen, even as they did from the Jews,
2 Th 1:8 dealing out retribution to those who do not know God and to those who do not obey the gospel of our Lord Jesus.
2 Th 1:9 These will pay the penalty of eternal destruction, away from the presence of the Lord and from the glory of His power,
2 Th 1:10 when He comes to be glorified in His saints on that day, and to be marveled at among all who have believed—for our testimony to you was believed.
Paul is promising the Thessalonians relief from persecution at the coming of the Lord in their lifetime (not hundreds of years into their future) when the persecutors/afflictors would be "punished with eternal destruction from the presence of the Lord."
The only nation on earth at that was said to dwell in "the presence of the Lord" was Judah/Israel.
This is the same coming/presence as in 1 Th 4:15-17.
If this relief did not come in the lifetime of the Thessalonians as Paul promised we would have to conclude that he was mistaken, or worse a false prophet.
He was neither.
The meeting in the air (figurative) was a spiritual event at the coming of the Lord not for the removal of the church but to dwell with it "so shall we ever be with the Lord":
There was/is taking the church out of the world, but there is deliverance:
John 17:15 “I do not ask You to take them out of the world, but to keep them from the evil one.
