What was the reason that Greece was not one unified country?

Answer 1

The Greek world comprised over 2,000 independent city-states, some of which joined leagues. They switched sides as their interests dictated. These city-states were spread around the Mediterranean and Black Seas, sowed in amongst other cities, petty kingdoms and tribal communities so there was no territorial continuity - the scattered cities went their own way and existed within the environments around them. They also competed against each other, being constantly at war, with ever-changing alliances. While something like the Persian threat brought some temporary cohesion, some cities were parts of the Persian Empire and provided parts of its war fleet, and some others actively sided with Persia against the southern Greek cities.

The Greek cities had many barriers to unity, not the least being that in an age of no telecommunications, aircraft or mechanical transport, communication and interaction was slow and ponderous. This combined with their internal divisions, made unity impossible.