Identifying the Restrainer of Second Thessalonians chapter two

Remember ye not, that, when I was yet with you, I told you these things? And now ye know what withholdeth that he might be revealed in his time. For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
(2Th 2:5-7)
If the Holy Ghost was/is the restrainer (what withholdeth) why did not Paul plainly write that in his epistle?