Part of this difference is due to how departments are funded: math departments teach a lot of "service" courses for non-math majors. So there are enough TAship

Part of this difference is due to how departments are funded: math departments teach a lot of "service" courses for non-math majors. So there are enough TAships to fund most students who want them. Most other programs do not have enough TA positions to support all the students they have. Source: I'm on the math faculty at a large public research university.