I'll answer you both since you both asked it. "Why a park only Marvel?"
I say "Why not?" Marvel has more than enough different completly opposite contrasting, extreme and basicaly 'different' properties from each other that you could make a whole marvel park based on these things and you would never know that they were all actual Marvel properties.
Thats the thing, people think of Marvel, what do they think? Avengers, spidermen X-men, and yes those franchises are all centralized in New England in the United stats (New York to be exact) so people go "How could those possibly be a park being as they're only in the New York.
But those of us that follow the comics know better, they know that Marvel comics spans from North America, to Europe, to Africa and Asia, from the North Pole to the South Pole and to the Moon and further, further beyond. And to even outside of the known Universe.
To base Marvel only on what is known by the mainstream knowledge of its characters based in New York, you miss out on the Technologicaly advanced yet still Tribal country of Wakanda, the primeaval oasis in the Artic that is the Savage Lands, the otherworldy homeland of Scientific exploration that gave rise to the Inhumans such as the Blue Area of the Moon, the intergalactic homeworlds of the Krees or Skrulls or even the Worldship of the Planet Devouring cosmic entity of Galactus. The techno/magical realm of Asgard and its 9 worlds.
I could go on all day about the many different worlds that Marvel comics has explored, each one unique as the next thats not "New York City"
Thats what people dont unerstand, and frankly dont need to enjoy, if a Marvel park would make people explore through the comics these places, its a bonus, but if these places are made the right way to capture the imagination, it doesnt matter.
And thats why is say a park can be "Only Marvel"