Why do people object to the license change so strongly despite having preference towards the proposed new license (CC 4)?

For me, when SE said they were changing the license on content I'd already provided, they showed me they weren't going to do what they promised. Put simply, the license change on my content shows me I can't trust them. Failure to respond to the objection showed me that it wasn't a simple oversight. Now I both expect them to break agreements we've had, and refuse to talk about or negotiate afterwards.

As others have said, the problem isn't the new license by itself.

These are the reasons we are complaining:

1. Abrupt decision: in the past, changes were announced before their release, so that it was possible to discuss the proposal and provide feedback. This time, instead, the announcement came out of the blue:

   Effective today, [...]

2. Illegal relicensing: lots of people think this change of license is illegal. This could even imply that they have already lost all rights to the entire content of the sites (!!!!!) that was posted under the previous license (that is, before 5^th September 2019).

3. Lack of response: when asked for clarifications, they haven't answered. What does it mean? I can see two possible reasons: they're guilty and they know it, and they are simply hoping that we all move on; or they just can't be bothered to give us an answer.
   Maybe both are true.

All in all, it seems they don't really care about this, as though it was a minor detail. Which is what I'd expect from your average start-up, run by amateurs. But from a company this size, with a legal office, and even hosting a site dedicated to open source projects and another one dedicated to legal matters? Seriously?

So why is the proposal still unpopular this time?

Because it's not a proposal.

It was done, with immediate effect, with zero consultation and zero meaningful interaction afterwards.

That is NOT OKAY.

Licences are important in our business, and SE showed an alarming and remarkable lack of comprehension of, and/or respect for, that.

That's the problem: utter contempt for their users (read: source of all their content).

CC-by-SA 4.0 itself is pretty good.

But you can't just singlehandedly decide that everybody who's contributed in the past now has their contributions licensed under something else. That's, like, literally the entire point of a licence.

Licensing is complicated enough, it's a necessary evil to understand enough of it to get around, but in the end I suspect most people really don't want to deal with the topic any more than necessary.

What this change introduces is uncertainty. There have been a number of arguments made on meta that to a non-lawyer seem plausible enough to cast doubt on whether this action by SE is on legally solid ground. I've no idea whether those arguments are any good, I'm not a lawyer. But SE is also not responding to those arguments, and isn't clarifying the legal base for the license change.

Now if I assume that SE didn't actually have the right to change the license, what exactly is the licensing state of all SE content? If I already reused content under the CC 4.0 license, but it turns out that SE wasn't legally allowed to make this license change, what rights do I actually have now? I've no idea what the answer is, and that is the scary part.

Licensing is complicated enough in the ideal case. A case where the license was changed in an illegal way is a situation that a non-lawyer can't reasonably deal with.

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.

Simply because it is illegal and a breach of copyright law.

It makes it illegal for other people to use the information from Stack Exchange. There is no license change. There is data that Stack Exchange might be allowed to use according to the ToS, but if anyone else copies that information and distributes it according to the CC BY-SA 4.0 it makes them a criminal!

I do also have a preference towards getting one million dollars. If you would rob a bank and then give me one million dollars, I would still object to it.

If the information on Stack Exchange would legally be licensed as CC BY-SA 4.0, via a lawful license change, I'd like that. But that is not the situation now.