The U.S. FedMob probably doesn’t like competition with their apps.
That is not it. It was bought by a Chinese company and all user information is therefore passed to the Chinese government and intelligence. Example: https://www.reuters.com/article/us-usa-tiktok-navy/u-s-navy-bans-tiktok-from-government-issued-mobile-devices-idUSKBN1YO2HU