The reason why Nintendo is better having its own park is hard to explain for me but I'll give you a lot of details.
Port of Entry: themed to an exotic and ancient port
Suess Landing: Themed to Dr. Seuss with attractions based on The Cat In The Hat, The Sneetches and other stories, and One Fish, Two Fish, Red Fish, Blue Fish, and his Seussian creatures; and nods to other books that didn't become attractions like Hop On Pop or Green Eggs and Ham.
Lost Continent: Themed to Greek/Roman and Arabic mythology and once had European medieval mythology.
The Wizarding World of Harry Potter: Hogsmeade: Themed to the Scottish village of Hogsmeade and Hogwarts School of Witchcraft and Wizardry.
Jurassic Park: Themed to Islar Nublar and Islar Sorna.
Skull Island: Themed to the 2005 adaptation of King Kong
Toon Lagoon: Themed to the creations Jay Ward and with attractions based on Popeye and Dudley Do-Right; again with small nods to other characters such as Rocky and Bullwinkle like Suess Landing
Marvel Superhero Island: Themed to Marvel comics with attractions based on Spiderman, The Hulk, and the X-Men and with small nods to characters that didn't make it to being an attraction.
Then comes Nintendo, and what does Nintendo have? Too many diverse and popular IPs to be in one land of the park. Because of how these franchises grown, not even nods would please the many fans who wanted to battle with Starfox or hunt Ghosts with Luigi; and not even retheming the largest land in the park and give the franchises sublands would be enough for many people. Not to mention that a lot of attractions are so popular that removing and retheming would break the hearts of the fans.
