If you remember at that time we were hearing from the left stuff like “people, not buildings” and “mostly peaceful protests.” Now we are hearing calls for law and order. We are hearing concerns about Antisemitism and the need to protect Jews on campus. I even heard Morning Joe call some of the protesters “white woke women.” While many on the left are very sympathetic to the cause of the pro-Palestinian protestors, their sympathy for the protestors' actions is decidedly less so. This was certainly not the case in 2020.
Now why the change in attitudes? I have two theories with quite different implications. The first is that Jews are not seen on the left as an acceptable target of score. While they are seen as more privileged than Palestinians, they are still seen as victims in society. So when they are the ones feeling threatened, the left will react. The potential victims of the 2020 protests were police officers and sometimes whites living in the suburbs. Those were acceptable targets, and thus there was no concern about their suffering.
My second theory is tied to January 6. Having used January 6 to signal being against democracy, it is hard to ignore protests today. I have even heard some leftists say that the violence on campuses resembles January 6. A major, probably the most major, argument progressives are making this presidential election is that we must save democracy and January 6 is the centrepiece of that strategy. The hypocrisy of doing that and excusing campus protests is not viable.
It is most likely that there is a combination of both theories, but one theory is probably more explanatory than the other. Which theory is more explanatory says something about how the left will treat future protests. If the first theory is more accurate than the next time there is a protest aimed at a group the left finds acceptable to stigmatize then we will get more “mostly peaceful protests” comments. But if the second theory is most accurate then there has been a seed change where there will be protests but no more taking over of cities for a couple of months and no more whitewashing the burning down of police buildings.