more often than 的用法？

Prior to 1975, union efforts to organize public-sector clerical workers, most of whom are women, were some- what limited. The factors favoring unionization drives seem to have been either the presence of large numbers (5) of workers, as in New York City, to make it worth the effort, or the concentration of small numbers in one or two locations, such as a hospital, to make it relatively easy. Receptivity to unionization on the workers' part was also a consideration, but when there-were large numbers involved or the clerical workers were the only unorganized group in a jurisdiction, the multioccupa- tional unions would often try to organize them regard- less of the workers' initial receptivity. The strategic reasoning was based, first, on the concern that politicians and administrators might play off unionized against nonunionized workers, and, s~on the conviction that a fully unionized public work force fficeffice" />


meant power, both at the bargaining table and in the legislature. In localities where clerical workers were few     in number, were scattered in several workplaces, and expressed no interest in being organized, unions more often than not ignored them in the pre-1975 period.


请问 “than”在这里做什么成份？ 由谁能给划分一下句子成分？我的理解是“unions”是主语，ignored 是谓语， 但有一点我不明白， 我认为这句话省略了did,


为什么不说：unions more ofthen than did not ignore them in the pre-1975 period.


谢谢