With inline quoting the relevant content of the conversation is immediately and easily discernible. You may Cc people with a whole copy of the thread at the bottom, but IME nobody bothers reading through. For one thing, it's in reverse order so very difficult to follow. It will also contain all the irrelevant parts of the discussion, which in a long thread will be the vast majority of the quote, thus making it even more burdensome to wade through. Part-and-parcel of inline quoting is pruning.
OTOH, inline quoting was always destined to fail. It requires that one purposefully and carefully edit the thread and reply in the appropriate place. That's asking too much of the vast majority of people. Perhaps it could have been preserved to a greater extent had Microsoft Outlook not effectively forced bottom quoting, but alas that ship sailed decades ago.
--
A: Because it reverses the logical flow of conversation.
Q: Why is top posting frowned upon?
A: Top-posting.
Q: What is the most annoying thing on usenet and in email?