Why don’t we use the same currency ?

Why do all EU countries use the Euro? There are the good, the bad and the ugly. Based on the theory of optimum currency area, the member countries have to give up their sovereignty. The factor of production like labor and capital has to move freely across the border to keep the single currency in touch. And if you have a shortage of currency, you cannot print it, or devaluate the exchange rate to find the new equilibrium. You will be a slave for life such as Greece. And more.

We DO use the same currency. All fifty states of the United States of America use the dollar, just as all twenty-odd states of the European Union use the Euro. Each state has its own laws on many concepts but we share a common language, currency and and national defense.

Because some countries' economy will suffer if their currency is replaced with a less valuable one. Just like Greece's drachma vs euro.

The Euro has been a failure. Here's why: 

Liberals create massive debt with reckless spending. 
When the debt becomes unsustainable countries essentially print currency to pay the debt. 

However, if everyone is using the same currency that isn't possible because other countries would get upset if you devalued their currency. 

So the country in debt has no real solutions They typically are maxxed out on how much they can raise taxes. Tax increases will only cripple their economy and in the long run produce less revenue. 

The only viable solution is to cut spending but libtards call that austerity and have a fit when you try to do it.   

That's what happened to both Greece and Spain. 

The transition to the euro was abused to increase prices.

We don't want to give that much power to one central bank

nations want to have the option of devaluating their own currency

and then there are less valid reasons, like "that's what the Esperanto-pushing jewish comunazi illuminati new world order wants"

and then there's chauvinism, like about the metric system