So that you understand my perspective, let me start by telling you that I personally prefer that we try to err on the side of leaving borderline questions open. Two of my most recent Meta posts, here on Super User and on Ask Ubuntu, have been regarding closures. The one on meta.askubuntu.com was also specifically about an "opinion-based" close.
And while your core question might be borderline (and I didn't vote to close it), I do fully agree with its closure in this case. There are just too many style issues with the post itself that put it "over the line." Please don't take that personally -- It's an interesting question, just (IMHO, and that of those who closed it) not for Super User. We're just not set up for "discussions", which is what I believe yours would lead to.
The problems I see:
If there were no USB-powered speakers, then it would be clear that there was a technical limitation that could probably be addressed as an objective answer. However, you point out that there are proof-points, but wonder why there aren't more of them. That likely points to a design difficulty, so that the answers would (I believe) really have to address, "Why is it so difficult to create a USB-powered speaker?" That starts to fall more into opinion.
You mention one particular brand, but worry that the audio quality might be poor. That might be an aside, but it starts to get into a particular brand, and could certainly be considered a "shopping recommendation", which would lead to a similar closure.
You mention another brand and specifically wonder why that speaker line doesn't have an update with that feature. Unless someone worked for that brand, it's impossible for the community to answer without pure speculation/opinion. So that's a question that should really be taken directly to the manufacturer.
So not only are these three (even if borderline) opinion questions in there, but that then adds the additional problem that it should be closed for "Lack of Focus". Questions should focus on one problem. Otherwise, one answer could be about one brand, another answer about the second, and other answers about the design difficulties or electrical-engineering challenges.
That just leads to "discussion", rather than clear-cut, objective answers. I do sometimes wish that there was a discussion-style board that had the same features and high-quality community and answers that SE sites do. But that doesn't mean that we can turn Super User into that discussion board.
Could the question be edited to make it worth reopening? Perhaps, but I really don't think so -- The first problem mentioned above alone makes it really challenging to elicit objective answers. Perhaps if it were something like, "What's the maximum theoretical wattage for a USB3-powered speaker?" That could then elicit the answers you said you hoped for here in your Meta question.
But that would probably be a better fit for the EE Stack (but I'm not an expert or community member there, so I don't know for sure).
I do think it's telling that there are no downvotes on your question currently. That, to me, indicates that the community doesn't think it's a bad question or an unhelpful one. Just that the votes-to-close mean that it doesn't quite fit here.