They had to move somebody from the AFC and Seattle made sense, they never really developed much of a rivalry with the rest of the AFC West (did Seattle fans even know that you're supposed to hate the Raiders) and the NFC West needed a massive redo anyway. They really went with the obvious plan that any 5 year old with a little knowledge of NFL rivalries could have come up with in 10 minutes, which is of course part of why it worked so well. Ignore some of the geographical weirdnesses like Dallas being "East" while KC and St Louis are "West" and Indy being "South" inspite of being North of most of the AFC "North" and these divisions make a lot of sense. Lots of traditional rivalries, good travel balance for most of the divisions, and most of the domes in the NFC where they belong.
It also didn't help that Seattle was AWOL from the post season for about a decade.