Because Instagram censors, shadow-bans and algorithmically supresses pro-palestinian content. Since anti-imperialist leftist were the first to condem the genocide, they have been made invisible on the platform. Ultra-left are compatible leftist, who don't threaten the empire. That's why they are left over on the platform, even, if some of them now too state the obvious about the genocide.
OP is stuck with "Down with Gazan imperialism" leftcoms.