The US Gov has a mandate to preserve and uphold democracy. Shuttering communication is prior restraint - an anti-democratic action.
Platforms have no mandate to preserve and uphold democracy.
In my dream world we’d get something like the rules we had, until fairly recently, restricting max broadcast media audience control in a given market for a single owner, but for Web platforms. Don’t like being limited to five million users or whatever? Then use a standard that puts control over curation and presentation in the hands of the user. Want to control all that, like all these awful platforms do? Then live with the limit.
Each of these platforms is fundamentally a propaganda platform—they're explicitly designed to manipulate people into buying stuff, and that capability is frequently turned to voter manipulation. The US government has decided that while US-based billionaires having access to such influence is fine and dandy, the CCP should not. So tiktok must be sold to a US owner.
Because worthwhile barriers to gerrymandering are difficult and complex to construct. Effective barriers would need to be overseen and updated by capable, uncompromised people.
Instead, it is easier for Gov to yield to its political handlers. There are lots of reasons for this; I think those reasons can be grouped together under one human failing:
No One Anywhere Wants To Clean Their Own HouseIf you want peace, you better prepare for war.
It is forbidden to forbid.
The necessary evil.
All that to say, we live in a complicated world, and beautiful ideals are only a direction to keep, never to be reached.
Of course, they don't HAVE to shutter. They can sell their interest in Tiktok and stay open. They have chosen not to do that thus far, and hence they have chosen to shutter.