Big Brother is the type of show where the host needs to be able to be good with scripts as well as dealing with unpredictable situations. It's one thing to have a great narrating voice and to be able to read an autocue, but the host also needs to be able to ask the housemates daring questions and to really put them on the spot. And Gretel was able to do that, Sonia not so much. Lets also look at the merlin incident, Gretel handled that tremendously well, Sonia would not have known what to do and would have waited for prompts from producers.