Brief - though partial - answer:

Japan is an independent state, which prints its own currency. Greece is a member of the EU, and its currency is controlled by EU bodies - It could not simply print more Drachmas to pay its debt as an (emergency) measure. The EU insisted it follow an austerity program, which only worsened the economic situation; Japan was not put under this kind of pressure.

Brief - though partial - answer:

Japan is an independent state, which prints its own currency. Greece is a member of the EU, and its currency, the Euro, is controlled by EU bodies - It could not simply print more Drachmas to pay its debt as an (emergency) measure. The EU insisted it follow an austerity program, which only worsened the economic situation; Japan was not put under this kind of pressure.

Brief - though not wholly complete - answer:

Japan is an independent state, which prints its own currency. Greece is a member of the EU, and its currency is controlled by EU bodies - It could not simply print more Drachmas to pay its debt as an (emergency) measure. The EU insisted it follow an austerity program, which only worsened the economic situation; Japan was not put under this kind of pressure.

Brief - though partial - answer:

Japan is an independent state, which prints its own currency. Greece is a member of the EU, and its currency is controlled by EU bodies - It could not simply print more Drachmas to pay its debt as an (emergency) measure. The EU insisted it follow an austerity program, which only worsened the economic situation; Japan was not put under this kind of pressure.