A combination of apathy, cowardice, and impotence. At least from my experience, this is what I've seen.
Apathy - Some individuals are perfectly content with the current situation or maybe even don't realize that the Union exists.
Cowardice - A lot of people just want to "do their job and go home," without putting a target on their back. So they don't file grievances, or really get involved with the Union at all.
Impotence - Some members have seen certain ironclad grievances fail or disappear one too many times to care anymore. They believe, best case scenario, that the Union is just weak. Worst case scenario, that there is active collusion between Union officials and management to sweep certain issues under the rug and possibly disparage others who make any noise.
As much as I hate to say it, the last one is what I hear the most from people. And I'm not entirely sure whether they're wrong to feel the way they do.