Steffy has already told him, kelly is her priority and regardless who is the father she's steffy's daughter. Steffy hasn't tried to use kelly to hold him and has told him she's perfectly fine on her own which she's shown as she's the one constantly seeing to her needs. Pam is itching to get over there and take care of that baby and ridge volunteers as well, even quinn said she's going to spoil her rotten, so kelly is no reason for liam to give up his one true love. Leaving now is best because steffy has already accepted it, too bad liam doesn't recognize that this time, steffy is a little less afraid of his leaving.
Everyone is going on and on about liam leaving because his side piece is still waiting in the wings. The twist is going to be that steffy is going to finally be the one to say bye and unlike liam and hope, when she walks away, she usually doesn't bother liam at all.