In form it approximates a classic "park" more than the sort of national parks you and I probably think of. I like the idea of diversity in our national parks and public lands. Some could focus on history and education, others on naturalism and ecology, others could mix and match as needed. Urban national parks seem like a fine idea and a way to encourage urban populations to explore more public lands outside of their cities too, eventually. To build a nation of parks and gardens would be laudable.

IIRC the story is it basically predates when they figured out exactly what a "national park" is supposed to be as distinct from other kinds of national-whatever, and it stuck.

Similar story for the one in Arkansas I think. Both would probably have some other designation if they were created today.

[EDIT] Oh LOL I was very wrong, that was just created. What I wrote was basically true of Hot Springs, not Gateway. Yeah that's a WTF.

Because Congress and Trump said so in 2018.