Why has it been difficult to resolve the debt crisis in the Eurozone?
The reason for this has been political. There are problems both within countries and among countries that make it difficult to resolve the issue.
Within countries like Greece and Spain, there is a problem based on past policies. These governments have been much too spendthrift with their money and have allowed various interests like unions to dominate their policymaking. Because of this, they have incurred huge debts and have a very hard time getting their people to accept changes that are drastic enough to help.
These countries, then, need to be bailed out by richer countries like Germany. But Germany is reluctant to bail the countries out unless they change their ways. The countries, in turn, are not willing to have Germany dictate to them. This makes it very difficult to arrange bailouts big enough to solve the crisis.