It's not (just) that it poses an economic threat to one of the biggest US companies (which as you said, I'm sure plays a big part in why it's suddenly relevant), but that it allows a government-influenced foreign media channel to influence policy indirectly by means of mass dissemination.
As for why now and not before, it's because of how apparent the possible effects are now that there's a very direct and widely spread channel that can pump OUT information, which is vastly more effective and obvious than passive surveillance through cameras or other hardware. (Also cybersecurity people have been calling out this sort of stuff for hardware since time immemorial)
This. TT's US popularity plus its fully algorithmic approach to content selection makes it potentially one of the most effective mass influence systems ever. People are easily influenced - especially young people - and the root of that being in China is a clear risk to US sovereignty.
I mean, look at what Xi and his allies would like to see happening in the US, and look at what's happening in the US today. Coincidence?
The framework is progress moves fastest with a global leveling and elimination of friction between markets.
Fundamentally I see this as American companies using government cheat codes instead of sharpening their game, just like with the Chinese electric cars 100% tax.
It stems from my critique of neoliberalism, the failure to invest in foundational societal systems that give an affordance to such positioning: education, mass transit, health care, maternity leave, preschool funding, scholastic enrichment, the stuff that most of Asia is great at we're lousy at.
Chinese people aren't like genetically superior, they just spent the past 50 years investing in their people instead of taking things away and fighting stupid wars.
And these are the chickens coming home to roost just like they did for Hungary, the UK, and Spain.
We've been fucking up for decades and racism isn't going to fix it.