Sylveon is the latest evolution of Eevee introduced in the Pokémon series. First appearing in Pokémon X and Y, you must level up Eevee while her affection for you is high, after which she will evolve into the Entwined Pokémon, Sylveon. The Fairy-type Pokémon has become quite popular, becoming the only evolution of Eevee to be featured in Pokémon Unite. Despite this, it is missing from the newly released Generation IV remakes, Pokémon Brilliant Diamond and Brilliant Pearl.
This probably has to do with the fact that Sylveon wasn't introduced until two generations after the release of the Diamond, Pearl and Platinum trio. Brilliant Diamond and Shining Pearl are mostly simple remakes of the original titles and are noticeably lacking in features that originated in later generations, including Dynamaxing and Mega Evolutions.
Interestingly enough, Sylveon didn't make the cut, but his typing did. Fairy-type Pokémon, which did not exist before Pokémon X and Y, can be found in the game. It appears that if Sylveon had been omitted because he was not present in the original game, his type would have been typed as well, but that is not the case, making the question of why Sylveon is missing one rather confusing.