I reckon he's retired because he's become a boring bowler in the last 6 months.
Johnson was always entertaining, it was either **** your pants and hide behind the sofa time as he broke fingers or found swing from nowhere to break batting lineups and have them jumping around all over the place, or sit back and have a laugh time as he struggled to find the cut strip, had the cordon scrambling everywhere to prevent 4 byes, and batsmen gorging themselves on a feast of short, wide rubbish.
This series he's done neither. He's become a nothing bowler, his pace is down so he's not scaring the batsmen anymore, but his control has largely remained so he's not getting hilariously flogged. It's been sad to watch.
Shame he's gone from all formats. I always thought he was a criminally under-rated ODI bowler because of his Test woes when he was at his worst.
And as a batsman I loved watching him. One of the best to watch when he got going IMO, played absolutely gorgeous shots and tonked them for ****ing miles. Loved his only ton in South Africa, got left stranded on 96* previously due to a hilarious collapse, looked like it was going to happen again so he made sure he got there by biffing Steyn for six.
One of my favourites. Test cricket is definitely a poorer place for having lost him, whether it was the bowling to the left and right enigma, or the wrecking ball of 2013/14.