The main economic countries have to work in unison on this for many reasons.
ie: One reason is capital flight.
FDIC insures your deposit up to $100k, lets say Ireland suddenly insures all deposits. Then you would see large depositors move their money to Irish banks as they are afraid that banks are failing.
If you had 1 million dollars in your bank, and lose faith in it surviving. If it failed you would get 100k back and lose 900k. Most folks would prefer to avoid that. So you move your 1 million to an Irish bank. Now your bank has to hand you 1 million dollars to transfer. Due to fractional reserves (they don't just keep your money in the bank, they invest it), they have to liquidate financial instruments to give you your money. People are not buying these instruments, they have a lack of faith in them. So they have to sell these at a loss to give you your million. Now due to banking laws, they do not have the minimum amount of cash on hand versus deposits. They have to liquidate more to make that ratio correct. Lets say it cost the bank 3 million dollars to give you your million. Not too big of a deal, banks can scrape up your million and not suffer.
Now 10,000 people with 1 million dollars do the same thing to the same bank. The bank suddenly has to repatriate 10 billion dollars to depositors....in a couple of days. Now the market is choking on the assets the bank is trying to liquidate, the price goes to 10 cents per dollar.... Now the bank has to sell 100 billion dollars, lose 90 billion to hand 10 billion back to the depositors. Bank dies (see Indymac).
Once this starts happening, people get nervous and start moving their money to safety (in this example Ireland), suddenly banks are detonating like popcorn in a microwave. It is a feedback loop.
Now repeat this on a national scale.
You can bankrupt a nation as fast as Iceland detonated. Iceland went illiquid instantly and is now broke, as a nation. They have 4 to 5 weeks of food left. Thier currency is now not accepted, they must pay in Euros. They are rationing Euros to get food shipped in. Suppliers will not extend them credit, they have to pay in advance. Last time I looked, Iceland is a big pile of Ice on top of volcanoes, not much in the way of making money other than cod fishing. They are in deep deep shit.
So by making these moves in unison, the countries/economies involved are saving one another from such capital flight. If one major player did not participate, then it would cause such "banking structural integrity" issues to arise in other countries.
This is just one of the reasons, it is kind of esoteric and obtuse but it has occured in the past so they knew they had to disuade such an event.
