Unified debt was explicitly forbidden in the EU contracts. It was one, of not the major selling point for the euro.
In a monetary union, the regions running a surplus enjoy a massive boost due the exchange rates being depressed by the regions running a deficit, while the industries of the deficit regions struggle to catch up, hampered by the lack of natural corrective forces of a floating exchange rate. If a monetary union has to work, the deficit regions have to be fiscally supported by the surpluses generated elsewhere, or they just spiral further and further into debt, unemployment and industrial collapse.