I'm trying to determine how the Greek managed to free their country from Ottoman rule as early as they did? According to Wikipedia they had an army that fought the Ottomans. That did not apply to other countries such as Bulgaria (they did not have an army).
Foreign help played a large role. Britain, France, and Russia intervened on behalf of the Greeks and destroyed the Ottoman/Egyptian Navy at the Battle of Navarino. This denied the Turks the ability to ship in reinforcements from Egypt (an Ottoman vassal with a large and well-trained army) and allowed the Greeks to have access to the seas. Loans and arms were also provided to the Greeks. The foreign intervention is largely what made the difference.
I any completely answer the question and I'll leave it mostly to historians who know.
But the Greeks have land that is VERY suitable to guerrilla tactics and that can help a revolutionary force. Look at Ancient Greek effectiveness against the invading Persians.
In fact, the Greek revolution started in the rural, mountainous areas of the Peloponnese.
Many Greeks, from 1453 to the early 1800s, had fled into mountainous areas of Greece to escape direct Turkish oppression in the cities. Over the years, the Turks had put down other revolutions in the Greek peninsula, but by the 1800s the Ottoman Empire was already in decline, and Greeks were in a revolutionary spirit with the American and French Revolutions both as precidents, and the Russian Empire being a strong Orthodox ally, it was a set of circumstances encouraging revolution.
As for why it was so successful, I'll have to leave that to historians who know better than me.
But one book I like is "A Concise History of Greece" by Richard Clogg. He does clarify in his book that the state of Greece (and his book itself) has little to do with Ancient Greece, as modern Greeks are more culturally attuned to the Byzantine Empire with the West adding a weird, plastic connection to Ancient Greece to the Greek people.