Because they rejected council of Chalcedon in 451AD which every other (every other that still exists, that is), Christian church apart from a few Arabic Christian churches that only have a few million believers total among them (such as Coptic in Egypt), accepted.

Every other Church (including Roman Catholic, Eastern Orthodox such as Greek, Russian, Serbian, Georgian etc., all mainline Protestant Churches etc.), believes that Jesus Christ exists in two distinct natures - divine and human - fully joined in one person or "hypostasis". Armenians believe that the divine and human natures are united in a single, composite nature within the one person of Jesus.

It is a Christological difference as big as possible while still staying "Christian" (one step of division further lies the Aryanism, which is already "certainly not Christianity"). Initially several Churches separated after the ill-fated attempt of reunification with so-called "Henoticon", but all others except the Armenian, either folded back, or simply became a tiny minority in their now-mostly Muslim countries. Armenia is the only Christian-majority country in the world that is still that way.

Think of it: dozens of wars and a millennium of mutual schism and hate happened between Catholics and Orthodox since the Great Schism of 1054. It happened over a difference much, much smaller than that.

This is why in fact Georgians and Armenians hate each other so much. They are the only two Christian nations in the world who are on different sides of the Chalcedonian divide and border each other.

This is also a reason why in Israel, Armenian population is bundled with Arabs, not with Christians, for statistical purposes. Because most people of their faith worldwide are indeed, Arabs.