I've seen one take that (maybe over-) simplifies to that there's a wave of anti-establishmentalism, and while the best party to embody that under FPTP (e.g. UK, USA) is the other big party, many European parliamentary systems have proportional voting systems that allow smaller alternative parties. And the alt right sells a load of coherent, anti-establishmentalist talking points.

Dutch examples: PVV? The problem is Islamic immigrants. FvD? The problem is the Deep State^TM^!

And although a lot of problems that we are experiencing (skyrocketing housing prices & cost of living, worsening labour conditions, millennials and gen Z/Alpha being worse off than their parents) are the result of the right, the left does not have a thing to sell as easily as the alt right has been able to.