Without a link to Kearney’s message, I can only guess as to why they were receptive. However, there are some general reasons that workers might be attracted to labor unions and their messages.
First, labor unions often advocate for better working conditions, higher wages, and improved benefits for workers. If Kearney's message promised to deliver these things, then members of the workingmen's party may have been receptive to it.
Second, labor unions can provide a sense of community and support for workers who may feel isolated or powerless on their own. By joining together with other workers in a union, members may feel more empowered to demand changes from their employers.
Finally, labor unions may also advocate for broader social and political changes that benefit workers, such as laws that protect workers' rights or policies that promote economic equality. If Kearney's message spoke to these issues, members of the workingmen's party may have been supportive of his ideas.