Discord being a US based company lets FBI access to its data much more directly. While TikTok still provides this data, them not being US based ticks off FBI, because there is the chance of chinese government interfering with the legitimacy of data that TikTok would provide. On China, the state has much more control over the companies than US.

The US can't even deal with a billionaire buying up a social media platform and influence elections; but China can collapse any company's success at a whim. Changing a legal data request from another country is certainly something they can do if China demands it.