Calculated cowardice, is my guess. The SPD leadership might also hope that a cautious stance loses them fewer votes in the upcoming elections. The BSW and AfD both cater to nostalgia and the fears of simpletons.
I personally don’t think they’ve done poorly in supporting Ukraine (Germany is a huge contributor, even if much of it is the boring non-kinetic kind) but he doesn’t have the strong leader quality that you want during such times. The initial Wende seems to have fizzled, too.
Did they finally stop paying their bills? Due to sanctions, I mean?