Perhaps because it is safer for the people who used to live on Cocoon to live in a place that floats, or so they thought On Pulse doesn't seem safe. Like Lebreau said "But on Pulse we had to deal with new hardships, like vicious monsters and a lack provisions."
But this is my opinion
And other thing that makes me curious or confuse was (SPOILER)why Hope called the new Cocoon Bhunivelze? Is there any explanation why Hope called it that?