(Warning: Armchair economist!)
Poorer countries in general have lower productivity (possible causes can be lacking infrastructure (roads, electricity, telecom, but also regulations etc.), lacking capital, lacking foresight/lacking capital to invest in future tech by the governement (e.g. broadband internet)).
This results - despite lower wages! - in higher unit labor costs.
If you form a monetary union with poorer and richer countries (like the EU), you have problem. The richer countries, despite higher absolute wages, will have lower unit labor costs. The richer countries can therefore expand exports at the cost of domestic production in the poorer countries (e.g. Germany vs. rest of EU).
If the poorer country wouldn't be part of the union, it could (among other things) lower the value of its own currency (or the forex markets would do that). Lower-valued domestic currency assists exports, which levels competition with the richer countries a bit.
This doesn't mean that there are no alternatives inside the current EU! They just haven't been implemented yet. Options would be
* way higher minimum wages in the rich countries (Germany was one of the last countries in Europe to implement them)
* implementing less union-hostile laws in the richer countries (resulting again in higher wages, but also for higher income groups not affected by minimum wage)
* More infrastructure investments in the poorer countries (there already are significant transfers, but more would help)
* A unified fiscal strategy. After implementation of the Euro zone, there were massive capital inflows into the poorer countries, resulting in overvaluation of the local economy. This was a big reason for the local problems of the last decade [1]
[1] https://streetlightblog.blogspot.com/2011/09/what-really-cau...