The key issue is that Europe is not homogeneous - There are a large number of very different cultures, languages and systems around the continent. You can't simply replace those with one without forcing everyone to learn a new language, abandon their entire cultural identity, etc.
It has also had a very tumultuous history, with several of the nations having something of a history of invading their neighbours, and imposing their will on them - and people tend to have long memories, and so tend to be unhappy at the idea of being told what to do by someone who previously invaded their country and murdered many of their compatriots.
Furthermore, even if you could get everyone to agree to have (say) a single language, which one would you choose? English is the most common second language across the continent, but the French would never agree to it being the standard, and similarly French and German would be rejected by several others - so unless you're going to for something like Esperanto, you're going to end up upsetting 20-odd countries with most of the options.