Too many variables to really know the answer at this point, but an interesting take.
Trajan was right at the end of his life when he did it, and his successor Hadrian withdrew from the Gulf just a year or three after the conquest of the province "Mesopotamia". Trajan had it goin' on, that's how he did it. :') Anthony had a real problem hittin' the sauce, and botched his attempt.
Hadrian had to be talked out of abandoning Dacia (Trajan's finest piece of imperialism, and regarded as the all-time peak of the Roman Empire, economically), but did pull back from Scotland along with Mesopotamia. Hadrian also spent a lot of cash building a cult of Antinoos, one of the boys Hadrian liked to, uh, romance, after Antinoos drowned in the Nile (got chomped by a hippo, or somethin').
I guess that was an unnecessary aside, eh? ;')