I think it has to do with time. People can come here from the same worlds but different times. So, they can be brought here from a time right before their death, but others can be after.

Except, well... Shiro's a unique case. He... [Keith took a breath. It was still hard to think about.] He died, but with everything that happened to him back home, we were still able to save him. So, he's kind of... alive... back home, still.