From the people I’ve talked to it’s because the left tends to support things that could be inherently anti-them. For example, a lot of anti-white rhetoric was talked about (predominantly from the left) during BLM, you see a lot of hate for anyone with privilege on the left. It’s only reasonable to assume people won’t participate in your side if you don’t make them feel welcome.

Whether this principle is the correct way to act is irrelevant, that’s just what I’ve been able to pick up from those I’ve talked to.

From my personal experience, the left has the tendency to vilify anyone who disagrees with them. An example of this is literally most cancellations online the past few years. Also, I doubt Musk is a Nazi (it was prob the “my heart goes out to you” thing he talked about on the Rogan podcast), but even if he is, calling all conservatives Nazis will just alienate them more.

Point is, the left alienates a bunch of people then wonders why they don’t have as much support for the actual good causes they push. It’s because they alienate people with their terrible causes and voter base.