As Zeromus_X said, he was looking for alternative energy sources and in fact leaves a message on Cloud's cell phone about finding a "big damn oil field." He ends the message with, "Make sure you're taking care of Marlene," or something to that accord. Basically, he's just a little busy and since Cloud and Tifa are already taking care of Denzel, Barret figures that they can take care of another kid and Marlene will have a playmate. I mean, you wouldn't want your child to be playing around oil drilling machinery, would you? 'Tis a little dangerous if you ask me.