Because Ctrl-C does not add the quote format, but the new ‘Copy’ button does.
[quote="Charlie, post:14, topic:285376, username:chapoi"]
Why would you use a click-action on desktop when ctrl-c is so ingrained
[/quote]
I think the problem is that the button doesn’t explain its function clearly. It’s not obvious that it does something different than just copying. Now, on mobile, I have two copy buttons. It’s quite confusing to have two options and, each time, I have to think about which option is my desired action (I know the result I want, but I have to think about which button achieves that).
On mobile, for me, quoting and cutting the quote worked perfectly fine. On desktop, I usually do it as explained in the Discourse New User Guide.
I see how this might be helpful for heavy users, but for new users (and even not so new), it creates a lot of confusion.