Because many people have been fooled by misinformation on large social websites, and it gives them a bad name, which is not in the companies' best interest. I don't really see this as a nanny state action, either. YouTube is not part of the state. From a libertarian point of view, shouldn't they have the right to censor what they want on their own platform? Yes, politicians are urging YouTube to take down the videos, but the key word there is "urging". They have no way to force them to take them down.
https://gizmodo.com/democrats-propose-section-230-changes-sa...
"Better do what we want, we're introducing legislation that will mess you up" is a pretty direct threat.
(There are also several other bills of a similar nature, that was only one example.)
They aren't saying do X or we will pass legislation about Y. They are saying do X or we will pass legislation forcing you to do X. The "widespread voter fraud" story is exactly the kind of extremist rhetoric that they are talking about writing legislation against spreading.