The entire reason to create cryptocurrency is to have a currency which cannot be manipulated by governments and central banks. This is supposed to be a form of "sound currency" rather than "fiat money", although as noted in another answer, cryptocurrency can be manipulated through other means.
Since governments and central banks actually desire a currency that they can manipulate (for example debasing currency [inflation] lowers the relative costs of carrying debt, something to think about next time you look at the size of the debts nations and political units like States or Provinces carry), a form of "sound money" is actually detrimental to currency debasement and other manipulations that governments might try to do. If you have a choice of keeping your value in fiat money or sound money, which would you choose to do business in?
This also explains the rather ferocious response of many governments to crypto currencies like Bitcoin, since the widespread adoption would have negative consequences in may areas, both from preference (people migrating to cryptocurrency), to taxation (cryptocurrency would be hard to find records of, making many types of taxation difficult) to data and record keeping, particularly when trying to record econometric data like GDP, the amount of money in circulation ("M" numbers) and so on.
So if the EU or any other political unit were to make cryptocurrency their unit of value, then there would be a great deal of instability during the period of changeover, and other effects (like a vast flow of "hot money" into the polity) would also take place. Eventually there would have to be a normal market mechanism to compare the value of the Euro (or whatever unit of currency we are talking about) to the Bitcoin analogue.