I had thought it was basically because it's monetary policy is determined by Germany, for Germany but it applies to every country in the EU. When other countries start faltering, their policy doesn't get adjusted because it's still about what's best for Germany and their economy.
A large, analogous economy like the US with it's regional state economies somewhat gets around this with a strong central gov't that can spread the wealth a bit to different, weaker states through federal spending, but stronger countries in the EU scream murder over bailouts.
The EU is either going to have to get much, much weaker or much, much stronger - as it is now it's untenable, and it's not just the fault of the "PIIGS" for being wasteful.