It's not exactly like that. The leader of new democracy could not find enough people to make a government. Pasok is now third party. However they the two parties that had Greece in this state for 35 years. Siriza is now second party and will try to find a way to form a government. Nobody declined, they just could not make it.
Siriza however does not agree with what the other two parties are sudgesting, and the people voting for it gives a message that the people want something different because they can't stand this anymore.