One reason could be nostalgia. When you exist in an international organization such as the EU, it’s expected that nations will forgo some of their local ways in order to adhere to the group. Germany did away with the Mark, France with the Franc, and the Dutch with the Guilder in order to assimilate themselves into the new scheme.
I bet if you went to any of those countries (or any Eurozone country today) and asked them for their opinions about the old currency, there would be some nostalgic comments. In fact, Sweden, Denmark, and the soon-to-depart UK have all kept their original currencies throughout their EU membership (with all of them joining the EU before the countries listed above).
A second reason could be the recent reputation of the Euro, given financial crises in Italy, Greece, and Portugal. When all are bound by a single currency and one member nation falters, the other nations are affected by any change of policy by the central bank, with individual nations powerless to take steps and control their own currency. This is a major reason for Poland’s resistance to join, as their ruling Law and Justice Party recently said. They want to wait until they are more economically advanced (85% of Germany’s economic size, they recently said).
Finally, a third reason could be that the Eurozone, or even the EU, might not exist in a few years. If it does survive this renewed age of populism and nationalism, it might look drastically weakened and look nothing like it does today. With this in mind, the upcoming French election is critical to look at. One of their leading candidates has vowed to remove France from the EU and to the Euro as currency. This is not isolated to France; one can find political parties in every EU country pushing this movement to drop out of the Eurozone.
Personally? I think individual national currencies are a good thing for identity, and economically to control the value of money easier. However, I find it incredibly convenient that I can fly from Berlin to Greece to Ireland to Finland and use the same currency in each country. There’s something to be said for that. We just don’t know yet if that something will convince the other members to join as well.