Why was Antifa designated a terrorist organization?

Simple answer: to provoke a cornered threat into a kinetic response,,, to determine/control the timeline regarding confrontation and total destruction...

Remember that active Military and National Guard assets were deployed around the country to liberal states and cities under a federalized charter prior to this declaration under the guise of immigration enforcement and the restoration of justice,, really it was just an excuse to move military assets into position for future operations...

understand that there is currently a WAR happening that will rise to the level of public awareness due to unfolding real time events...

In a nutshell the centralized control system is becoming aware that it is targeted for total destruction and now being pushed to fight back/respond so that it can be flushed out and totally destroyed by its own actions...