One difference is temporal.
Taking the groupings in Allure's answer you'd find that a number of people moved from the "support" group to the "oppose" group over time.
The reasons are various. The WMD claimedclaims seemed, to some, credible at the timein the beginning, then that turned out wrong. Some people really thought democracy could be brought to Iraq. On the US side, the long grinding casualty count was not apparent in the first 6-8 months - that gradually ramped up the revulsion, along with places like Fallujah and scandals like Abu Ghraib.
To add to that that for those operating in the public sphere, past support for the war is now best disavowed but quite a few were on the record supporting it.
On the flip, the Russian invasion seems to have struggled having much of a "support" base outside of its own population from the start.
As a comment states, temporal drift cuts both ways. Meduza.io claims (Nov 6th) that Russian propaganda has made some good headway in Germany towards injecting a "Russia was pushed into it" sentiment (note however that we disagree somewhat about the reason).