Thing is, those who are against immigration are much more motivated. This is generally how politics goes, that a motivated minority can eventually get their way against a less-interested majority. As great as it is, that was largely what happened in the other direction with issues such as gay marriage.

For those who are anti-immigration this is often the only topic they care about, or they care much more about this than anything else. On the other hand, those who are pro-immigration are largely either mildly-favorable towards it or they care much more about other issues.

Personally I think that left-leaning parties in Europe basically have two options currently:

-Fight against anti-immigration stances and then lose power.

-Focus on other topics while getting tougher on immigration.

It might not be the smartest move to be anti-immigration, but unfortunately in a democracy feelings are usually more important than facts and one of the biggest issues with the left is not being able to accept that.----