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.