It's an interesting question - What is restraining the man of sin from being established - at 2 Thess 2:6 ?
Several years ago I got interested in this question and studied it a lot. The answer seems to be that TIME is the restrainer, that it wasn't yet the right TIME for the "wicked one" when Paul wrote his 2nd letter to the Thessalonians. The GNT version of 2 Thess 2:6 below seems a bit more clear on this than some of the other versions -
2 Thess 2:6 GNT - Yet there is something that keeps this from happening now, and you know what it is. At the proper time, then, the Wicked One will appear.
In other words, the events of the past 2000 years had to play out first in order to lead up to the TIME of the 3rd temple to be built in Jerusalem on the temple mount so that the "Wicked One" can be set up there and claim to be God - see Temple Mount