While both the socialists/IWW and the mainstream progressives thought that the capitalist system in place in contemporary America was abusive, they had very different ideas about how to change the system.
Socialists and the IWW rejected capitalism completely. They wanted to institute government ownership of factories. They wanted to do more to have the government redistribute income.
By contrast, the mainstream progressives only wanted to reform capitalism. They were willing to accept the idea of government ownership of a few key utilities such as electric companies, but they were not willing to go farther than that. They wanted the government to enact laws that would curb the greatest excesses of capitalism. However, they were not interested in destroying the basic capitalist system.