EDIT: heh, fastposted by NotASenator.
I'm guessing that it's because it would be almost twice as much work to implement every change and feature in both themes. From what I can see, the "classic" theme is the one which is having all the work done on it; the "modern" theme is just a temporary placeholder theme, one that is entirely functional for when the in-progress "classic" theme isn't. Once all the planned features and fixes are implemented, then the fully-featured theme can be duplicated and changed to make sub-themes (like a darker version) without having to actually code the features twice. I'd guess that the modern theme is different enough from the classic theme that you can't always just copy-paste sections of markup or code from one to the other.