TikTok is a Chinese company bringing in mountains of data on US citizens, including the ability to influence what people in the US see on a daily basis. The Chinese government would never sell that kind of leverage to anyone, let alone to Facebook, which is banned in China.
TikTok is incredibly popular in the US and there isn’t really a huge similar precedent where the US takes a page from China’s playbook and blocks a social media network completely. If banning TikTok was reasonable, and I’m not sure it is, I think it makes a lot of sense to be cautious about setting bad precedence.