I don't believe there is a determination about vocals. Many people refer to female vocals as being one of the characteristics of the genre. But it can't be taken that seriously. It's just a tendency, not a necessary element. I agree with the predilection, and believe the choice is related to a "musical counterweight". I explain. There's a lot of darkness inside of trip-hop. Many times, when it's not preponderantly dark, there's at least an influence of the darkish atmosphere. On an ambient like that, female vocals do promote a beautiful contrast, as between the sharp and the deep, metaphorically: the good and the evil... the beauty and the beast. Similarly, sad songs do sound even sadder when performed by female voices. Even if this can't be taken as an element of trip-hop, there's in fact a preponderance.