As it is written, it appears to be a conditional:
Because a militia is necessary, we need to arm the populace.
If the initial condition no longer holds, why should the required consequence still be a general requirement?
And I think we can show, that today, the USA does not require a militia to ascertain its freedom. Therefore, the right og the people to keep and bear arms no longer stands upon its founding principle and has to be reassessed under the changed conditions.
To get back to the original topic:
Guns were never really needed by private persons in Europe. Therefore a market for guns never developed. There is no conservative basis in the people's gun ownership. Also, where the US had few wars actually happening inside its borders in the last century, at the end of WW2, the last century saw a number of large scale wars happening in Europe, in places where people lived. Guns were often the symbol of the oppressor. Even during the French revolution, it wasn't the guns that made the difference, but rather the people's momentum and the economic situation. Guns are an aristocrat's toy and a soldier's killing tool, not a symbol of freedom.
Add to that, that Europe is mostly densely settled, which leads to more potential social problems, where guns would only worsen the situation.
Also, Europeans have different existential fears, that remain cultivated by conservative media. Personal security just isn't much of an issue, that anyone talks about, but social issues are more en vogue. Furthermore, because guns were never widespread, there's much less criminal gun ownership, and people are less confrontational about many situations, because they don't feel like they can just shoot their way out. Because criminals know this, even if they have a gun, they are less likely to use it in a preventative manner, and a simple robbery doesn't turn into a more brutal crime because of a sudden escalation of the issue.