This would probably need a more thorough analysis but I think three things are crucial here: the salience of immigration, the infighting within the SPÖ and the media landscape.

1) Immigration is a highly salient issue in Austria, which mostly helps the far right as other parties either co-opt their stances or don’t have a convincing counternarrative. 2) There is/was substantial infighting at the top of the SPÖ the party was not very united behind Babler to begin with.