Because good cop, good cop in general would cause the suspect to think that the cops are easy to push around, manipulate, and get pass. They won't say a single thing because they believe the cops are too nice to even retaliate.

In terms of the Malnosso, if they were good cop, good cop, then we'd be back home already and we wouldn't be experimented on. Why do we have to be experimented on just to get home? If they play good cop, bad cop, it also serves to make the residents feel as though they have hope and that allows the Malnosso to experiment on us even morel, because we don't have our guards as up as when it's bad cop, bad cop. Am I making sense?