1. Andrew Strauss retired in 2012, who was a very good opener and captain for England. When he retired, a spot opened up which made an uncoming Joe Root get a shot in the Test team. Joe Root has since become an ATG for England since.
Root actually debuted instead of the retired Paul Collingwood at #6. England had struggled for a few months to fill that slot, bouncing around between Morgan, Bopara et al.
Strauss's slot still hasn't been filled yet. There's a reason Cook has teamed up with no fewer than 19 opening partners in the last few years alone.