Short answer: because doing a good thing for a bad reason - particularly when it establishes a bad legal precedent - is often a very bad thing in the long run.
I fucking detest TikTok for a variety of reasons (largely around the societal impact it has), but playing the national security card and then not applying it consistently (e.g. meta, twitter, insta, etc are not getting NEARLY the same level of scrutiny, despite very similar ways in which they influence society at both a national and global level) is a recipe for trouble, because that card can be (and often is) played for esoteric bullshit reasons. And I fully expect the incoming admin to use esoteric bullshit reasons for pretty much anything they possibly can.