Danubian Legions and Parthian Wars

[Centurio]

I was looking at the map of Roman world in AD 117 and realized that after the conquest of Dacia, the province of Upper Moesia with its two legions (IV Flavia Firma and VII Claudia P.F.) lost its status of frontier province. What is a mystery to me, why Roman government used to sent Pannonian legio II Adiutrix to the East during Parthian wars of AD II instead of one of Upper Moesian legions, stationed in a rear province? Both Pannonias (Lower and Upper) remained frontier provinces and as Wilkes stated in his article legio IV Flavia arrived to Aquincum from Singidunum to replace II Adiutrix which was involved in Eastern wars of Lucius Verus and Septimius Severus. Why Emperors didn't send IV Flavia or VII Claudia directly from Moesia to the East? What explanation can be given to this decision?