Because they will kill this ******* currency union from the inside..
well the US has a currency union too so in principle its not a bad idea. The solution to most financial issues is the Republican Balanced Budget Amendment. LIberals hate the idea of fiscal responsibility so we have all this endless trouble century after centuryl.
A common currency along with a union of states is not the same as we have in Europe.
The dollar ( whatever one may say about its current situation is not the point) was the common currency from the beginning (few little exceptions). You have of course different economical situations in your 50 states, but not as serious as in the EU with its different countries, languages, mentalities, political and legal systems and historical interests and burdens.
The Greeks actually never had a viable economy at all, except the short period when Onassis & Co. dominated the oil tanker market. The Portugese are in a similar situation, only a tiny bit better off is Spain. After they lost their colonies in Middle and South America and Africa their foreign trade is more or less restricted to olives, goat cheese and tourism.
The French are industrialized, like the Italians, but never did well because of lacking work ethics and a massive patriarchalic structure at the workplace.
The Brits will never let go of their splendid isolation, the transformation from industry to financial services (just moving other peoples money around) had hit them pretty hard. But they still have a close tie to their former empire colonies which is in serious conflict to the EU common market.
Left over are Germany, Austria, The Netherlands and the Skandinavians who could have indtroduced a common currency without any problem.
You must understand, in Europe you cannot simply move to another state if it would be of any advantage like in the USA, neither as employer nor as employee. Legally yes, but the economy does not consist of a few "cosmopolitan" Managers and Banksters who change their penthouses like their underwear.
First, you have a language problem. This is serious, did you ever try to use one of the European languages (except England) in a daily profassional way?
You have different legal situations. Land ownership in Portugal or Spain, Greece and Italy as well, is a horrortrip to any foreign investor. You need laywers and translators, and of course tax consultants because the differenes in the fiscal systems are a nightmare.
I was once offered a job in Austria. You know, they speak the same language, have the same mentality (ok, in your view as American at least

), as a South German you feel like home.
Then I studied the tax situation, the social insurance conditions, and other items like the operating cost if I took my car with me. The result was a "thank you, but NO".
This job was merely 100 kilometers from my place in Germany, to be more specific and illustrate the European mobility problems in the economy.
Lots of Europeans own a house. You don't leave it like in the US, and just buy another one at your new location. It takes you the hell of a time to build one, cost you a fortune (for the 100k you can buy a house in the US you get a garage for your car here) and the mentality is absolutely different. If you build a house you die in it.
So, to boil it down, the different countries not only have differences in their economical performance, it simply does not smoothly work together like sprockets in a gear drive, even if the sprockets have different speeds.
Normally the different currencies and interest rates act as buffer (or grease) to compensate that.
If you take away this grease your gearbox will eventually crash. This is what happens now.
You see, just for the PIIGS states a brief explanation of the foreign trade with us.
Greece, or Spain, issue bonds (trash papers) to their central banks (now even to the European Central bank), who in return transfer fresh Euros to the government. This money goes to the public, and the people buy a Mercedes Benz (to be a bit graphic). Ths Euros without any countervalue are at Mercedes now, they take them to their bank, which gives it to the German Central Bank.
This balance is called Target II. Actually the German central bank now has a claim against the Greek, which is in foreign trade terms useless because Greece has nothing to sell for this self printed Euros.
In short, we gave the Greek the money (or accpted trash money) to buy goods from us, but the money we got was either already ours or is completely worthless. So for the trade deficits of the PIIGS states with Germany we are working for free.
You must understand, this cannot end well. You know why the US banks and government advisors were such in favor of the EURO when it was to be introduced? Because they are smart and knew that.
At that time Germany alone was the export nation No. One in the world. A functioning common market in Europe, without this ******* currency, was the nightmare of the US economy. because it would have been without any competition for a long time. Worse, it could have made close ties to Russia with their endless, vast ressources.
Any Question why Mario Draghi became boss of the European Central Bank? The former manager of Goldman Sachs? Goldman Sachs, the bank that fudged the Greek balances before entering the EURO zone by granting them a loan under conditions that pushed Greece down the financial stairs in the first place?
All this is not just a common currency. This is a complete sophisticated set up.