It’s because the left has failed to improve people’s lives and over the last decade has alienated much of the population with identity politics and virtue signalling.
If you want the left to stop the right turn things are taking, you gotta drop the communism, the identity politics and the virtue signalling and ficus entirely on workers unions and environmentalism. Those are the past 2 areas where the left can not only hold full sway but has a lot of public support from those who are put off by the earlier mentioned things.