Good question.

First of all, a country should have the desire and will to be a part of the EU to become a member. This is not always the case and some aspects of it can be considered as negatives, both in practical and ideological terms. Broadly speaking, a country is giving up part of its sovereignty to become part of the Union. Not all are willing to do so, say Switzerland and Norway.

Then, along with some technicalities, the EU wants to make sure that the applying country is really an established democracy and has a fully functioning market economy. This is, of course, not always down to beauty and shininess and also has to be sustained for a considerable time to make sure sudden U-turns are not possible. It is a risk to consider, especially given the fact, that the Union does not really have the power and mechanisms to intervene if that happens.

But at the end and most importantly, it always comes down to selling the very idea of the enlargement to societies of member countries. The final approval comes from their governments and the political context is not always suitable for that. Croatia in particular seems to suffer from that and this will probably continue for the years to come.