The rumor has been that the original contract required all resorts getting the initial Super Nintendo World land would also have to expand with Donkey Kong and Zelda later. (That's what made me feel pretty confident that Hollywood would get DK eventually, at least back then anyway.)
Singapore's SNW was approved and announced AFTER the initial announcement, so I have been working under the assumption that it is not under the same contractual obligations, if these are even true. Especially given the small scale of that park.
There was a rumor that Japan, which is strapped for space as well, would prefer not to have to do Zelda, which at the time was being designed as a large ride that would take up a lot space, and someone suggested maybe they would rather do Animal Crossing instead. No clue as to the validity of this rumor though.
This came just after USJ partnered with The Pokemon Company, which I bet was a bigger deal for them to act on soon than Zelda at the time. At least they are getting DK next year. After that is anyone's guess.
But Nintendo was the one who wrote this contract with Universal with this supposed clause, and the are a co-owner of Pokemon, so if they wanted to skip Zelda and move straight to Pokemon next at USJ, they could just get with USJ to amend their agreement, which happens all the time with these types of theme park arrangements.
In other words, we really don't know anything lol