Sinnoh contains the most important legendaries canon-wise. It has the Lake Trio which represent Knowledge, Emotions, and Willpower.Next, the Creation Trio, which control space, time, and the alternate dimension. And finally it has the God of the Pokeverse, Arceus. These are the most important Pokémon in the entire series and the other legendaries in the future generations cant ever live up to that in terms of importance in the Pokeverse. Sinnoh should have happened after Unova, Kalos, and any other future generations. It should have been the last generation in Pokémon.