TikTok is being banned for a bunch of different reasons all added together.

Republicans agreed to it, among the other reasons, because Democrats will get the blame for it and it will hurt Biden at the election.

Republicans and Democrats supported its ban because of sinophobia. It's a big, successful business in America, there's scaremongering around what data it's collecting or ways it could be used to manipulate people's opinions—ByteDance did not do itself any favours by coming out and telling all its American users to tell their Congresspeople to vote against this, which was spun as a demonstration of that power.

They also support the ban as part of an ongoing backlash against "big tech". Republicans are angry at big tech because they think it's too liberal. Democrats are angry at it for being addictive, abusing monopolistic powers, and other quite legitimate issues. The problem is that neither party is very good at actually dealing with the problem, so they just lash out wildly at whatever comes along that looks vaguely tech. Not realising that in this case, that will give way more power to Meta and Alphabet.