I think it's ultimately just up to people and their feelings / how significant they feel a game is/was to put the resources in to preserve it. I also feel like some games are clearly more notable than others that a consensus would agree with.
Mario Bros 3, Zelda, WoW, Skyrim... I could go on for hours I'm sure, but certainly some games have a much larger impact and are more historically notable than others. Maybe it's partially subjective, and I'm not sure what the solution is. Do we legislate these things must be preserved?
That seems pretty extreme. Do we ask nicely? It seems most companies won't listen if there's no profit there. I'd be interested in hearing other people's ideas on that and how to make it happen. I think it'd be a real shame if in 20 years, given all our technology, these things were lost to history somehow still.