Cyrus was called 'the great' because

He was known as 'the great' because he created a whole new alphabet, known as the cyrillic alphabet

As the "father of the Iranian nation", he was the first world leader to be referred to as "The Great ". Cyrus founded the first world empire - and the second Iranian dynastic empire (the Achaemenids) - after defeating the Median dynasty and uniting the Medes with the other major Iranian tribe, the Persians.