Why was Alexis Tsipras able to form a government again after the September 2015 election?

Alexis Tsipras was able to form a government again following the September 2015 election because his party, Syriza, finished first in the polls with 35 percent of the vote.1

This victory awarded Syriza an additional 50 seats in the 300-seat parliament, bringing their total to 145 seats. By maintaining their previous coalition with the Independent Greeks, who contributed 10 seats, Tsipras secured a parliamentary majority, allowing him to return as prime minister without needing to expand the coalition to include other parties. Tsipras had resigned in August 2015 and called for these snap elections to seek a fresh mandate from the Greek people after facing internal party dissent regarding the terms of a new bailout agreement.1