There is a compelling reason to name Bran as King of the Seven Kingdoms - and it ties in with the series overall theme of power. Despite suffering heavy losses as a young boy in the early season, the Stark family have continuously proved resilient when it came to surviving (RIP Catelyn and Robb). Compare that to the utter destruction of most of the other royal houses in the Seven Kingdoms, and it starts to become clear why this show began as the story of the Stark family contrasted with other royal houses of the Seven Kingdoms.
As Vox's Todd Vanderwerff puts it, George R.R Martin is a big fan of "circularity", whereby events double back on themselves. One of the obsessions, as well, of the creators was the idea of the impossibility of just leadership because we are all limited by our human traits - including passion, intelligence and heart. Bran, though, isn't really human any more.