Though I think the reason is very clear, at least their official one. If you are a China based company the CCP by law has access to your data and they have abused access to sensitive and private data time and time again. Just because nothing has happened yet doesn't mean we shouldn't be proactive against China.
And I wish Congress would do something about it. The Chinese government deserves so much more then a TikTok ban and Congress could give the ban a more stable foundation then a executive order from a controversial administration.