It is a major part of the cultural identities of both the Georgian and Armenian peoples. Keep in mind that they both like to claim they are some of the first nations to adopt Christianity as a state religion (Armenia in 301, Georgia in 325), and each has a distinct sect (Armenia has the Armenian Apostolic Church, while Georgia has the Georgian Orthodox Church; both are loosely related to Orthodoxy in general), and have their own Patriarch (analogous to the Pope). Thus with centuries of religious influence intertwined with the very idea of what it is to be Armenian and Georgian (and indeed, in Georgia at least, there is a strong current of opinion that non-Georgian Orthodox are not really Georgians, despite nearly 20% of the population not of that faith, including a 10% Muslim population; I can't speak for Armenia), it was not easy for them to convert, especially when both the Armenians and Georgians fought so strongly to retain their ethnic/national identity in the face of constant invasion and attack.

Edit: In response to some replies here, I apparently have exposed my lack of knowledge on the intricacies of Christianity and the composition of the various churches, and that apparently the Armenian church is not simply a branch of Orthodoxy, but much more complex than that. Seems I have some research to do on the subject.