Has taken a look at BSW's Wikipee page.
There it says they are "eurosceptic". So basically the existence of their party means that even on the left there is support (demand) for ideas like "euroescepticism" and "anti-immigration" in Germany. Those people just don't want to vote on AfD. And their political bias to the left, and the labels which comes with the political palette, is more important to them, than the stance in these questions. So if noone represents "euroscepticism" and "anti-immigration" on the left, then they are willing to vote on parties that want to abolish states, centralize the EU, and import endless amount of muzzies and PoCs, because they are not willing to vote on the right.
I write the two terms within quotes because they don't really mean what these words imply.
"Eurosceptics": they still want EU, very few if any of these guys think that their country would do better without the EU, they just want to preserve more from the sovereignty of their state, they don't want EU bureaucracy to rule over their country, and over the decisions their own political system and their own people do
"Anti-immigration": this is more like a palette from those who want to regulate immigration, who want the keep the right for their country to pick whom they allow in or not - to the guys who would pack and export the immigrants zusammen, no matter what