The reality is that Europe has been in a state of ideological vacuum for a while. The hard right (including most dramatically fascism) really failed between 60-40 years ago as a movement. The hard left failed 25-35 years ago as a viable movement. The old left-right paradigm continued. In many places it's been centrist policies combined with left wing rhetoric. How long can that be the paradigm?
All that's left is the reality that governing bodies, parties and paradigms have been under fire for 5 years now. Political populism and dissidence sells. All that's left is to find a rhetoric that appeals to young, radical or reactionary pockets of political capital.
To me, one of the important takeaways from marxism/socialism is that forces of political stability and instability dictate history to a large degree. I'm very far from Marx on his determinism, but I agree with the broad brush idea. If the current paradigm is both unsustainable and unchangeable, pressure will mount. Technology is a force that Marx never truly reckoned with, but it contributes to the process. If people don't benefit from the system or suspect they could benefit more from some other system, they'll try to kill the old system.
UKIP make almost as much effort to recruit from the hard left as they do from the hard right.