(Edit 2: read the bill, it also bans American companies from offering hosting services to a company that is banned through the law https://www.congress.gov/bill/118th-congress/house-bill/7521/text)

Arguably, if the app isn't easy to obtain then the cost of all the US-based servers would become an enormous expense. All US customer data is on its own US-based infra hosted by Oracle. Migrating all the US data elsewhere would also be an enormous expense. Server infa for 170 million Americans on an App is not gonna be cheap to keep running, esp since even if tiktok tried, the best they could do is get apk's to android users. iOS users are SOL.

Given how iOS dominates the US still and only a small portion of android users are comfortable manually installing apps from non-store locations, why would they go through the effort to stay around for a fraction of the previous user base.

Its a perfectly uneerstsndable business decision, and its one they may be making on the hopes thay the ban will get reversed shortly after its put in place. Its also perfectly understandable to not want to sell the US-based component of the App when they still operate in plenty of other countries, including China, and the sale would devalue what they retained.

(Edit: and while their web offering has improved over the years, they probably are assuming a similar drop of userbase since only so many would be willing to move their usage to a web app that is not super easy to use for capturing video or handling notifications)