1. Regarding huge number of grads and few people voting - wiki and other aggregate sources of info will only tell you the total number of students listed as "enrolled" at the school. Not all of them taught in the previous five quarters. That actually reduces the number of grads eligible to vote drastically.
2. Regarding the comments linking union to money. It is by no means guaranteed that a union can increase the salaries paid to grad students. And in fact most grad students (at least at the PhD level) survive off the stipend not the TA/RA salary. As far as I understand the union has no bearing in the stipend amount. There is also the thing that UChicago is cash strapped. It doesn't have the liquid assets to increase anyone's salaries. (Look up the aggressive campaign to sell UChicago owned buildings in HP if you are curios).
3. A union, as an organization of people, by definition caters to the average contributor. In a factory where the workers provide similar enough service that 'an average' is still a meaningful concept, a union can do some good. In my opinion, in a union of all grad students across all departments, 'average demand' is meaningless. Each department can't even agree internally on what their students need, so I'm not sure how a union will find common ground among all the departments. On the one hand limiting the number of hours in a work day sounds good, no? But ask a science grad - they will likely complain that they no longer have time to finish their experiment in time. Giving students a choice in which class they want to TA sounds good, no? But ask a student in humanities - they will tell you that seventeen of them apply for the same spot that only one person can have so they would prefer assigned positions not chosen ones. Etc, etc, etc.