In the UK the primary motivating factor for people joining a union is because they have been asked to and provided with the forms.  That requires a unionised workplace.  

Many employers (illegally) actively discourage workers from joining a union and workers can get blacklisted, especially in the construction industry, for union activity or membership.

Construction - blacklisting

In March 2009 the Information Commissioner's Office raided the offices of the Consulting Association and exposed the existence of a blacklist containing the details of 3,213 construction workers.  They also found that the blacklist had been used by over 40 UK construction companies.