While I think that some of what frashaw said is true, I do think there is another reason: Klei is trying to optimize profits, which I am not hating on btw.

Making the two games that much different incentivizes people to spend the extra money getting both. I bought dst initially because I played with other people, but now that I play alone, I might consider buying single player eventually just to try it out. Single player has shipwrecked and hamlet while dst has turn of tides. Obviously there are different characters and nuances between the two versions as well.

Klei has made it attractive to buy both because they are just enough different from one another!