Why would a country not use the Euro if it is a member of the European Union?

the french wanted it, they would not agree to German reunification* unless a common European currency would be created.

*after ww2 Germany was divided into 4 sectors, one under American, one under british, one under sowjet and one under french occupation. The three former formed west-Germany, the latter east Germany. those four powers had to agree to reunification before it could happen.