The fact of the matter is, if you are working class, you are far more likely to be threatened, and this includes the quality of earnings of your livelihood, by unfettered imigration than other classes tend to be, though of course the middle-classes are not entirely immune. (The wealthy and the professional classes, on the other hand, are often sheltered, albeit for different reasons...and perhaps slightly less so in English-speaking countries)

This isn't particularly complicated...and some mainstream left parties, notably the Socialist Party in the Netherlands, which had a positively break-through election in NL's last parliamentary elections (not speaking for the last), have caught on and are tuning in to the working class voters they purport to wish to represent.

Perhaps it is that in other countries, the mainstream left parties have been captured by the professional classes who have been largely sheltered from the effects of immigration? Small wonder, in a country like the UK, with no proper outlet for working class voters (they are given short shrift by all parties, really) that a UKIP comes in to fill the void.

That the void exists is not an indictment of those who would waste a vote on UKIP. It is an indictment of the lack of representation of working class voters for over a generation in that country.  

The Hun is always either at your throat or at your feet. Winston Churchill