I can not tell you why, or the reason why. I can only guess that it is because the EU wants better control with anibiotics and maybe also with what gets poured into the sewage.
Practically, to legally buy fish medications here, you need a prescription from a vet. This in itself is difficult, because partly very few no vets know much about fish treatment (3 in the whole country and they don't take consultations they work in zoos). The reason they won't prescribe medication is that they can be charged for wrongfull medication if they prescribe something that was not needed. Now, even if you can get a prescription from a vet, for treating your fish, you can't buy the well known fish medications because manufacturers haven't gotten their medications approved by the national authorities. This means that the best a vet can do, is find similar substanses in medications for dogs or cats, and prescribe them in the appropriate dosages.
Luckily, it is possible to import medications from other EU countries as long as it is for personal use (no selling to others).
How are they available in other EU countries you ask? This is the weird part. The EU is, as far as I know, not targeting home fish keeping. They are after the fish industrry who breed fish in large scale for consumption. Therefore, the EU has allowed member countries to exclude aquarium fish keeping from these regulations (we could once buy medication off the shelf at pet stores). Each country decides if the want to use the exception or not. So the country where I live, has chosen not to, but the neighboring country has, so I can order them from there, online and have it shipped. By the time the right medication arrives though, most fish that are in need of more treatment than salt, have already passed the point of recovery.
I am sorry for the long rant, but I have been quite involved in this, even talking with politicians, local authorities, and even manufacturers and EU representatives.
There is not much to be done really and it frustrates me that fish are being differentiated so much compared to other pets. Imagine a vet only being able to suggest treating an infected dog wound, with salt or let it die.
We had a similar situation with pet rabbits/bunnies, as they practically also can be bred for consuming. Medications were being regulated so much that vets were not allowed to prescribe them to pet rabbit and guinea pig owners. This has been overturned though, and is not a bit more relaxed.
I personally think EU has done some great things, and this seems great too. It is my country's implementation of them that is terrible.