It’s a list of groups that typically have very high support for communism/socialism. Historically, as supporters of the cause they have a militant culture, an attitude of entitlement, and an expectation that they will form the power structure of the new society. This ironically makes them the biggest threat to the new dictator (the guy who is convinced that he alone is just enough to control every aspect of society). Thus they become the first people on the hitlist of the new society they created.