In the first place, ANTIFA isn't even an organization nor a well defined group. Rather, it's a position, that is being against Fascism. It would be like declaring the civil-rights movement a 'terrorist organization'. Or the environmental movement.
In order for Trump to be seen as a 'victim', he needs a 'villain'. The White supremacists and Neo-nazis who marched in Charlottesville are more of an organization than is ANTIFA, but you never hear Trump talking about declaring them terrorist organizations yet they're responsible for many more deaths in this country than any 'group' of anti-fascists.