For humanities students, the department or division makes a lot of decisions that affect grad students, including the assignment of TA and RA positions, providing funding for conferences and other travel (e.g., to archives). Issues like fairly distributing TAships and keeping class sizes reasonable are things that a union could address. Their advisors were literally....advisors; some of my friends met with their advisor monthly or even less and their research was usually related to--but not an integral part of--their advisors' work.

In contrast, science PhD students (especially in biomedical fields) usually work for ONE professor and their work is usually a piece of that prof's overarching research plan. The prof (through grants) usually provides funds for salaries and research-related expenses, rather than the department. The department has very little effect on their lives; bad interactions with their advisor are the main source of misery. It's less clear to me how unions could solve this. They could prevent students from being dismissed for not working insane hours, but they can't force advisors to actually help students do and publish experiments, write them useful letters of recommendation, or things like that.