Because they want to reform capitalism rather than abolish it and you can't work with someone like that on the project of abolishing capitalism. You can try to radicalize them and convince them to help. But to just take them and their views as allies won't work. The goals aren't aligned. Socdems may now be considered more left than they used to be, but only because the right has shifted yet again. This doesn't mean their goals are compatible with socialism/communism. Eventually current Democrats will be far left, but you can see how that doesn't make them good allies.
It's possible to work with people who have different ultimate goals, that difference just has to be resolved one way or another before that end state is reached.
Because they want to reform capitalism rather than abolish it and you can't work with someone like that on the project of abolishing capitalism. You can try to radicalize them and convince them to help. But to just take them and their views as allies won't work. The goals aren't aligned. Socdems may now be considered more left than they used to be, but only because the right has shifted yet again. This doesn't mean their goals are compatible with socialism/communism. Eventually current Democrats will be far left, but you can see how that doesn't make them good allies.
It's possible to work with people who have different ultimate goals, that difference just has to be resolved one way or another before that end state is reached.
Historically those differences have been resolved in favor of anti-communism and MORE capitalism