Maybe part of it was that there was a widespread desire to be irreverent, and the hosts mixed a lot of history and politics in with their media critique.

But the bigger reason why r/CTH took off is because r/communism sucked and r/anarchism sucked and r/socialism sucked and r/latestagecapitalism sucked. As soon as r/CTH became known as a nonsectarian space that upheld progressive politics but no specific banner, and started to develop a reputation of its own, including making fun of reactionaries instead of banning them, it began to really take off.