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Since it does a whole lot more than 3.5 ever did. So obviously some of the functionality from the few discarded % of code, have had less priority to be added back.
Seriously, KDE SC is something like 4-5 million lines of code, do you really think the KDE developer(they are not that many) designed, developed, tested and debugged all that code from scratch in the few years since KDE 3.5? For most, in their spare time at that.
It's true that the KDE developers are awesome, but they are not quite on that level of super humans.