According to this newspaper article, it was for procedural reasons only. The motion was brought forward jointly by the Conservative (CDU/CSU), Social Democrat (SPD), Liberal (FDP) and Green groups. The Left group would have liked to formally co-sponsor the motion, too, but was excluded upon the Conservatives’ request. This due to the Conservatives’ policy of “no cooperation with extremist parties” (in their view, the Left and the AfD).
From the article (my translation):
The Left in particular considers this an affront. “I recommended that my group abstain”, Bundestag vice president Petra Pau of the Left tells taz [the newspaper]. They would likely have supported the motion if the other groups had allowed her group to weigh in on it, says Pau. “We were past that point already. I hope we can return to how it was handled during the previous legislative period.”