Quite simply because it has been imposed on something which is not anywhere close to being an optimal currency area.

FORBES: The Inevitable Failure of the Euro

And for a common currency to work it must be applied across something which is at least close to being an optimal currency area.

FORBES: The Inevitable Failure of the Euro

There are a number of good reasons to think the optimal long-run inflation rate is close to zero.

FORBES: Monetary Easing Has (Almost) Nothing To Do With Deceiving Consumers