Because TikTok not only has the tracking potential of BigTech, but it is a serious competitor to some of them.

It's just protectionism, like banning the Huawei phones (not talking about the infrastructure, that's different) or banning DJI drones. It just sounds better to mention national security than to say "we don't want others to do to us what we do to them, and we don't want them to compete with us".