The biggest problem I have with the change is the communication around it. This is a direct change in the legal relationship between the users and the site: communicating the change should be the top communication priority. It should have been communicated by e-mail, blog, meta-post, and banner. I certainly haven't received an e-mail about it; the banner was used to advertise some gimmicky newsletter, but not the licence change; and I'm not even sure that there was a blog post about it. One out of four is poor.
Moreover, the meta-post was IMO badly worded. It took me at least half a dozen readings to understand that the change to 4.0 was intentional: I initially read
This change follows our last Terms Of Service (ToS) update where we inadvertently introduced a point of confusion: the new ToS links to version 4.0 of the CC-BY-SA license in support of defining the use of a Creative Commons license where the previous terms linked to version 3.0 of the license.
to say that the change to the ToS was inadvertent, but that having made it they were going to commit to it.
So with those two negative first impressions, I started reading everything I could find about it on meta. There wasn't much from SE, and that's a big strike against it. As noted in the question, we went through a big brouhaha about licence changes in 2015. SE should have learnt the lesson from that experience that licence changes create uncertainty and confusion. Even if they felt it strictly necessary to impose the change without advance warning (which in itself is problematic), the initial announcement should have made every effort to explain why relicensing old content was permitted. They still haven't explained that, despite being asked.
I think that the TL;DR would be: the reason that the change provokes strong negative reactions even from people who like the new licence is that it's yet another instance of SE's approach to communication conveying a strong subtext of "We take you lot for granted". People who contribute value expect to be valued.