Look at the Cuomo / Mamdani primary for NY mayor, the DNC really tried to kill the socialist campaign. Most primaries like that don't get that much support against the "old guard" among Democrats, so most of the time, the actual good non-centrist option gets swept away like dust. The work that's needed to push a socialist through the primary is so much more than "you just need to get out and vote" because those like Cuomo have so much more weight to shut them down, it's so rare for that to fail like this.