Titus Flavius Vespasianus is called the man of lawlessness because he permanently put an end to the practice of the Law by destroying the Jewish Temple in A.D. 70 thereby making it impossible to fully follow the customs of the Law of Moses.
2 Thessalonians was written before Titus's men destroyed the temple, supposedly, and the lawless one was already around by then (
2 Thes 2:4). And why would a Roman have been the lawless one?
The Romans concerned themselves not one iota with the Judean God. A Roman leader would not have proclaimed himself to be this God (
2 Thes 2:4) and then to seat Him in the Roman pantheon. Roman citizens often enshrined their emperors as gods, and some of the emperors even demanded such enshrinement. But if for some strange reason a Roman leader broke character and proclaimed himself the Judean God, he would not have violated any of his own laws. And not being Jewish, the Jewish concept of lawlessness would not have applied to him, either.