Marleybonians "took over" Marleybone because it's their home world. Marleybonians "took over" Celestia and Krokotopia because they are explorers and researchers, so they would naturally want to seek out new worlds. I agree with thunderbird, the environment is totally different than Marleybone.
While the beginning music and characters are kind of Marleybonish, trust me, it doesn't stay like that. I will admit that I am a bit sick of the dogs since we already saw them in Krokotopia and, naturally, Marleybone, it does get more different as you progress through the game. - Brianna Thundersong, level 54 diviner, currently questing in Stormriven Hall.