Почему наша родина изначально называлась Великая Русь? Кратко и понятно, заранее спасибо)

In ancient times the Russian Federation was often known as Rus'. The name is historic. It send us back to the tenth century. Those times there inhabited Varangians and one of their tribes' names was Rus'. The first documental mentioning of Rus' was in 991 in one of the Russian— Byzantine agreement. Its capital was Kiev. Now this word itself doesn't exist anymore and you can find it only in historical books, but in the name Russia some part of it remained. The poetic name of the country came from Old East Slavic. I am sure that I can call my country Great Rus' as it has a very long and great history. It was baptized in 988. That time no one could even imagine about existence of America!