The D:OS steampackage contains both Classic and DOS:EE, so no that's not it. It's very easy to have a single purchase enable 2 games.

Benefits of a single install: The reviews stay I suppose. That might be it? Less listings in your library of games.
Benefits of 2 games: People don't have to install a ton of deadweight, another set of Trading cards (= money for devs), another listing under "new releases" being PR. People get a +1 wink

So... beats me. It seems the 2 games system got way more benefits than the current one, so yeah, I also wonder why they did as they did.