One of the reasons is effects quality I believe. With CSD you have one buffer which holds your entire window's contents, including window decorations. The compositor (Wayland) can then transform/warp/etc your window however it wants (for all those pretty Compiz-like effects) without doing to much special stuff to get rid of the aliasing between the window contents and decorations.
There may be other, more technical reasons for it, but I'm unaware of them. Also, as someone else mentioned, Wayland can do both.



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