The Reason Why The European Leaders Trying So Hard to Keep Greece in the Eurozone

Nigel Farage has an eloquent speech about that:

Greece outside of the Eurozone may even be an inspiration for Portugal, Spain and many other countries.

There, you have it: the first reason, which we've mentioned many times here, is the belief that the whole banking sector of their respective country will collapse, and the second reason, is that the whole of the Euro might collapse, as countries start seeing the benefits of defaulting and leaving the single currency.