Neither side had the power to defeat the other outright.
The Athenians were dominant at sea but unable to assemble sufficient land power to defeat the Spartan alliance, staying behind their city walls, conducting amphibious raids, and collecting tribute from its empire to finance its fleet and supply the city.
The Spartan alliance was dominant on land, but did not have the capacity to either breach the walls of Athens or match its navy at sea.
This changed when Persia provided money to Sparta, which was then able to expand its alliance fleet and hire the best sailors, so Athens was matched at sea as well as under seige at home, and unable to collect the tribute from its empire which financed its fleet. Defeat of the Athenian fleet in the Dardanelles ended its naval capability and ability to continue the war.