The same reason that harassment and slander are crimes. There's always limits to what you can "say" when what you're saying isn't just unpopular but actually damaging.
The deepfakes aspect of this law could be considered akin to slander (an untrue statement that damages someone's reputation). Deepfakes are basically fake/fraudulent videos that can be analogized to "untrue speech". If I were trying to defend this law against a First Amendment challenge, I would use this approach. I would also expect that once deepfakes are sufficiently common, people will defend them on the basis that "it's just for fun — everyone knows they're fake." That would be akin to the defense against false advertising for subjective statements like "Tastiest Pizza In Amador County!".
I would note that the deepfakes law is unlike harassment crimes, which are not illegal because of what you're saying, but rather the person to whom you're saying them.
Even if you are found guilty of harassing someone verbally, you would be free to make the same statements about that person to someone else (assuming they don't also constitute slander, by virtue of being untrue). Basically, there are very few limits on what you can truthfully say about someone; harassment law only applies when you're saying such things to an unwilling listener.
One problem we're going to face is that the defence that "it wasn't me it's obviously fake" will apply to all forms of media too. I never said "grab them by the pussy" it was fake, I never raped that person on CCTV it was fake, and the existence of deepfake tech provides reasonable doubt in many cases in which previously the doubt was limited and a sound conviction could be found.
Or satire if you put a disclaimer that it's not the person depicted. Would you want a cartoonist arrested for the involuntary porn of painting trump naked with a micro penis?
It's a tort, not a crime. The individual you defamed can sue you in civil court over the damage you do to their reputation, but the US government can't prosecute you for it.
