"After that debacle I honestly cannot understand how anyone with even an iota of integrity can vote for them again"
Well, there is actually a very old song about them, here in a new version (that is also already quite old by now)
https://www.youtube.com/watch?v=8vFL0QWxugI
But the answer is probably lack of real alternatives.
The slogan "Wer hat uns verraten? – Sozialdemokraten!" has a rather long history. Coined by communist during the 1918 November Revolution, it referenced the social democrats at least tentative support of the suppression of the communist revolution and the murder of Karl Liebknecht und Rosa Luxemburg.
During Weimar times it was popular with both right-wing and left-wing forces. The Nazis - who sent social democrats to the concentration camps - also used it in their propaganda.
As much as I disagree with many of the things today's social democrats say and do, it's worth remembering that this slogan was popularized and predominantly used by anti-democratic forces. It's not a particularly fair thing to throw around carelessly.