This is a very complicated question, which is made even more difficult because of the mythology which surrounds the rise of the 'polis' (Greek city-states) and the democratic trend.

Athens did not become a democracy instantaneously, it developed over centuries. According to conservative 4th century BC Athenians, democracy was their 'ancestral constitution', and its founder was undoubtedly Solon. According to Herodotus, however, the founder was Cleistenes. In reality, Solon started a process of democratisation which would be developed further and further until it reached its zenith just before the conquests of Alexander the Great.

Solon's reforms emerged from a crisis in 594/3 BCE which he sought to resolve by 1) depriving the self-styled 'best', the ruling aristocracy of Athens, of their monopoly of political power and to throw open the major offices of government to the wealthiest Athenian citizens; 2) he aimed to give a voice- including the registering of decisive votes- on some major public issues to the poorer citizens.

In 508/7 BCE Cleisthenes again reformed Athenian politics and most importantly introduced the administrative council of 500, comprised of well-off farmers.

The other major candidate for the invention of Greek democracy is a reform bill associated with Ephialtes and supported by Pericles in 462/1 BCE, which set the stage for what we generally think of Athenian democracy. And even then, it does not compare very well to what we call democracy today.

If we want to answer the question 'why Athens', we need to think about factors beyond individual reforms. I believe that the process of democratisation was inherently linked to the rise of philosophy- in particular- the Sophists, who helped to create a culture in which speech and language was revered. If we analyse the political thought of ancient Greece (and in particular, Aristotle), we find an overbearing pre-occupation with speech and oration. Rhetoric, the 'art of persuasion', was considered the great political faculty. If we want to understand why (as opposed to just how) democracy developed, we need to look at Greece's strong connection with language, and the emphasis placed on persuasion, rather than force.

This emphasis upon speech was to be found throughout Greek culture, so there was a great deal of support for Athens in many areas; however, this would wane as Athens became a major Mediterranean power and started influencing other Greek city-states by threat and force.