It's because the left are bad at propaganda (maybe on purpose). Really stupid ppl need rituals. The right provides those. The left doesn't want to manipulate ppl and doesn't want to be a cult or look conform. So they don't unite under one banner.
The right has chants and new racist drinking songs every year. They like dressing up in uniforms, wave qestionable flags. This appeals to ppl who want to belong.
If a large group of leftist parties could agree on things like that, the differences would just be factions within a united group.
You know, like maga hats. Vague. Harmless at first glance. No difficult history. Unites the sheeple.