I was southbound in the UK at about that time and heard traffic being advised that Maastricht were not accepting traffic via KOK. It was rerouted via REDFA.
To answer your question, consider the effect of a thunderstorm (or series of them) sitting in the middle of the narrow airway at KOK. If no-one will fly through it then they will go round it and this may put them in conflict with traffic which would not normally be the case. Multiply that by each aircraft wanting to take a different route around the storm and chaos rapidly ensues. Safety dictates that traffic shoudl be stopped from getting to this chaotic state.
Stopping traffic is not a decision taken lightly, so ' one thunderstorm ' was probably not the situation. Just because you didn't see any weather doesn't mean there wasn't any. As for 'the whole of Europe', how do you know? I certainly picked my way aroundseveral huge buildups between the IOM and NOKIN, which I believe still count as being in Europe.
If you don't like this answer, wait until your next request to avoid weather is refused and see how you like the ride...