I don't think it was the atmosphere or the unit choice, it was just bad storytelling. The Pacific wandered all over the place and you wound up never caring about anybody. I watched every episode, and I couldn't tell you about a single character in the entire series.
This sums it up, it wasn't the setting or anything it just that you didn't spend enough time with any one unit. In BoB you were with easy since their training till the end of hostilities, so you knew the characters pretty well by the end. In the Pacific you are with too many people for too short of a time.