Greece already received something like 50% haircuts on the debt. Politically
Greek people voting on something has no influence on the rest of the population of the EU. If Greece had more of its debt written off then other countries would want the same. Hence it wasn't politically possible for those on the other side of the negotiations to give this to Greece.
The only real hand Greece had was to simply refuse the bailouts and refuse to pay the debts. In that case they would have been cut off from international bond markets, they would have had to renounce the Euro and start using a different currency.
All that was possible, but the Greek government could see the obvious - it would have been far worse than what was on offer.
Politically, because he was trying to negotiate against austerity measures with zero leverage and no realistic alternatives. Given the consensus on the requirements between the countries footing the bailout and the general public (outside of Greece), it was clear from the start that his position was unworkable.
The only real hand Greece had was to simply refuse the bailouts and refuse to pay the debts. In that case they would have been cut off from international bond markets, they would have had to renounce the Euro and start using a different currency.
All that was possible, but the Greek government could see the obvious - it would have been far worse than what was on offer.