Why did the speaker of the statement call moscow the third rome?

The initial idea of this name was from a monk named Filoféi, where he explains

The Roman Empire was "the first Rome"

Constantinople was "the second Rome"

Moscow is "the third Rome"

Its purpose was to highlight Russia as the last glimpse of Christian civilization, most of which had succumbed to heresy as ROME and CONSTANTINOPLE.                        

the correct answer is history and tradition.