While everyone knows that a certain US intelligence agency insisted on having a backdoor built into a well-known operating system and thereby rendered it more vulnerable than it would otherwise have been, the fact is that a European OS would not add any resilience at all - an OS is always going to have points of vulnerability if a machine running it has to connect to other machines. All a Euro OS would do would be to create huge cost and even more headaches for systems support personnel. Remember how Jobs fatuously boasted that the Apple OS had never been hacked... and within 30 minutes of that stupid claim, someone hacked it very publicly.

Creating a Euro OS would simply mean more expense and wasted time for Euro bureaucrats who'd be forced to use it while everyone else very sensibly ignored it entirely. There's no benefit, there's a non-trivial cost, and therefore the entire idea is bad.