I don't think it has much to do with unionization at all so much as that there is generally an overabundance of students applying to graduate school. Combined with the fact that the NIH/NSF more or less set the wages anyway, there really isn't anywhere for a union to lean. After all, unless you could organize a strike of a significant portion of the graduate student populace in the US, I doubt Congress or the NIH/NSF will take any notice.