Prior to 1975, union efforts to organize public-sector clerical workers were somewhat limited. The factors favoring unionization drives seem to have been either the presence of large numbers of workers to make it worth the effort, or the concentration of small numbers in one or two locations to make it relatively easy.
what does the "The factors favoring unionization drives" mean ?
A union "drive" is a campaign of sorts. It involves a large group of employees banding together and convincing the other employees that forming or joining a union would be beneficial to all concerned. In the past, if employees felt they weren't being treated fairly, they could often easily be convinced to unionize. Today, however, the overall climate in the workplace has changed, and to some, a union is more trouble than its worth.