I wanted to do an elaborate reply, but no time, so here is a list of bullet points, no claim to them covering everything

• "regime" is just the mediator of inner-bourgeois conflict, different faces, same kinda guys
• media monopolies, they can and will eviscerate even harmless imperial lapdogs like the usual greens if necessary
• in the west: a relatively content population due to imperial kickbacks (which are in decline, but capitalism-imperialism encourages people to kick down first to shore up priviledges, see fascism) that would rather have 10000s drown in the mediterranean than have higher taxes or less cars or less oppressed gays or other upsets in the status quo
• enormous economic power of the bourgeoisie: capital strikes cant be fully ignored by elected people, revolutionary situations may be different,
• intellectuals are either being marginalized or in the pockets of foundations or other philanthropy-washed bourgeois money
• bourg. democracies are set up in a way to protect private property, you would have to capture all three departments of political power to do anything significant and then you still had a hostile bureaucracy that will sabotage you at every turn
• social power: The superstructure is not just the state, but also "civil society" aka social clubs, associations etc that are mostly led by "managers" (promoted tokens etc) or actual bourgeoisie
• US imperialism is still a thing

Brazil's case is kinda special, also Lula had to make significant concessions iirc and that may still end up biting him in the ass. Not sure, Bolsonaro played his hand really badly