Why did tyranny decline in ancient Greece because there's only a person who seized it in control?

Answer 1

Tyrants were appointed by popular acclaim to run Greek city-states when ruling aristocracies abused their position and got involved in their own internal disputes over power. The tyrant was supposed to govern for all.

There were many successful tyrants who ran clean regimes in their city-state. However the disappointed aristocracies were always waiting to reclaim power, and seized opportunities to do so when a tyrant fell out of favour with the majority of the citizens. Other forms of government, such as restricted-franchise democracies and experiments with radical democracy, provided alternatives, and the number of cities with tyrants shrank, but it was always a fall-back position to appoint a tyrant when other forms of government failed, in order to head off civil war.

A similar sequence appeared later in Rome, with dictators appointed to resolve electoral and war crises.