It's a pity that they don't get in touch here why.
It's worth noting that you didn't actually ask for that. This is the message people see:
"2020-06-06 - Layout changes Today we have activated a new cleaner layout. If you prefer to use the old layout, you can change it in your profile. Feedback? Use the Forums."
If you feedback as to why people switch back, you need to be clearer. After all, why would anyone take the time to provide feedback if they can simply switch back to the former version.
In a future version (where also this forum will get a huge version jump forward) there will be no way to activate the old design. i can't please everybody, but i'll try my best. So give me your feedback, and as exactly as possible what is not as good as before in the new version, so i can work on it.
This is the message you need to give everyone because it gives them a concrete reason (the old design is going away) why they should provide the feedback.
It's also probably a good idea to tell users why you are changing the site. Personally, I'm a "if it ain't broke, don't fix it" person. I respond better to changes if I'm told why the changes are made.