Well, first keep in mind that Avarayr was a classic Pyrrhic victory. It wasn't even a conquest - it was the putting down of a rebellion led by a disgruntled supreme commander, Vardan Mamikonian (in his mid-sixties, no less). The Sasanians paid a huge prize merely for maintaining the status quo. In 481, the nephew of said supreme commander, Vahan, started another rebellion, that was put down in 484; the background to this rebellion was among other things (such as renewed religious persecutions) Armenian levies being drafted to fight the Hephtalites far in the east.

Then the Hephtalites kill Peroz I, the great king, triggering a succession crisis between his sons, his heir apparent Balash, and Zarir. This causes the occupying forces in Armenia to withdraw, freeing Vahan's hands, and Balash requests his help in securing the throne.

The price for this help was the Nvarsak treaty, and Vahan was soon appointed not just supreme commander, but governor of Armenia.

There are potentially more complex ideological forces at works; Touraj Daryaee in Sasanian Persia notes the increasing power of the Arsacid noble families of the East and the increasing fascination with the mythologized past of the Kayanids following Peroz' operations in the East. However, there are more than enough immediate material causes to explain Balash's treaty with Armenia.