Libertarianism literally originated on the left. Right wing neo-libertarians were a creation of the mid 20th century. A product of the anti socialist panic/red scare of the 50s. where all socialist ideologies were demonized where possible. Or usurped and undermine in the case of Rothbard. He's also the guy that came up with anarco capitalism. Because nothing says you're against large over arching rules and systems. Than being a hypocrite and making an exception for your favorite system.