Democrats hate progressives. The two would be separate parties in a sensible democracy, but in the legacy first-past-the-post American system, splitting the party while Republicans exist is political suicide. There are more progressives than center Democrats, but they are not as united as the core establishment Democrats are, which made any one of them irrelevant in primaries up to now. RCV has given them a real chance for the first time. For example by giving Lander votes to Mamdani in the instant runoff. The moment RCV got implemented, long-serving Democrats started suddenly getting kicked out in the primaries. This terrifies them, so Democrats will continue to fight against RCV in the future. Just look how both Cuomo and Adams will try to commit political murder-suicide now by running as independent in the general just so that the “official Democratic party nominee” would not win.
There has never been a law that someone selling something must offer the same price to everyone. Outside of some government regulation, like banning discrimination based on a few specific protected groups under the 1964 Civil Rights Act, government-set energy prices on state-granted monopoly electrical grids, annual rent increase percentage caps, etc. merchants have always been free to set any price on any product or to any customer.