When the original Pokemon Diamond and Pearl was released in 2006 (or 2007 for everyone outside of Japan), Nintendo and Game Freak introduced a new feature called the Global Trading Station (GTS). The GTS has given players the ability to trade their Pokemon with other players around the world via Geonet and the Nintendo DS's internet connectivity via Wi-Fi.
You would think that since Pokemon Brilliant Diamond and Shining Pearl are faithful remakes of the original Gen 4 games, GTS should be included in the game, right? Unfortunately, but the answer is a resounding no.
Trading, fighting and interacting with Pokemon Trainers on a global scale has become so advanced in the past 15 years since the Wi-Fi functionality was introduced in gaming systems such as Nintendo DS and Wii, the functionality of the GTS did not it was most needed. Game remakes are all about improving graphics from scratch and deciding which features should remain or disappear, and Game Freak and IILC have decided to drop the GTS for Pokemon Brilliant Diamond and Shining Pearl based on online game progress. Their decision to do so was ripped from the Pokemon Sword and Shield playbook, in which GTS was by no means included.
Instead, Nintendo wrote in its day one patch note that it will introduce the Global Wonder System (GWS) to a facility in Jubilife City in a future update. The company hasn't shared too many details on the GWS, but will allow various types of Link Trades in the building that once housed the GTS.