The TOT at the DLR was much more polished and enjoyable than the DHS version. Why get rid of the better version, especially when it fits the story of the park. DHS is a Studio park, so it makes much more sense to add a Marvel ride here as it fits the theme of a Studio park. The 5th Dimension Hallway also yields more opportunity for sets/AAs, giving this attraction the room it needs.
And why not just build a new original attraction? Obviously, that is preferred, but WDW tends to be a bit cheap and it is much cheaper to just retheme TOT and use the existing footage from DCA's retheme than it is to build from ground up. Also, the DHS Tower is showing its age and needs an update. Currently, it's the best thing in a bad park, but once Star Wars Land opens and Pandora fleshes out, TOT will stand out as being very dated technology-wise. We know WDW hates updating tech, so a retheme gives them the excuse.
Also, the Sunset Blvd area throws off the balance of the new park, in my opinion. Hollywood entrance into a world of movies. You have Star Wars, Muppets, Toy Story, and Sunset Blvd. Which one of those doesn't belong? I love TOT, but it just makes too much sense for DHS to follow suit.