Why waste time on a waste land? I too wish they would have stayed longer, and frankly managed the episodes more that they could have less bullshit episodes and more substance, but oh well.
They wanted a new home, Earth wasn't that. They are running low of food and need to hurry to find a new home.
And Three wanted to stay on Earth, did you not see the end of the episode last week? I think that is fitting to her character.