Social democracy may sound like a good idea in theory, but capitalists can always retaliate against making the populace less exploitable through stuff like capital strikes. And since that system still respects private property, the countries that practice the system have no choice but to either obey the capitalists face economic downturn, like what happened two years into François Mitterrand’s presidency. Plus a lot of social democracies were imperial empires, such and Britain and France, which is how they fund their welfare programs. And even socdem countries that weren’t benefit from unequal exchange, like Sweden or Norway.
Also communism has the people to be the checks and balances of the ruling party. Unlike what happens under any form of capitalism, consent for a party to rule can’t be manufactured by private news corporations under communism, nor can the bourgeoisie favor certain political parties by funding them.