This is surely an Offense and Defense zoner game.
If that is the case, why is that the kings experience is only calculated by the attacks carried out as an offense. Should not kings also gain experience by defending?
One school of thought is that since we carry the king in offense only, we gain exp. points for the king. Hey, the king (only) does not go on the attack, he carries troops with him. The same troops defend his kingdom.
Now, the game is moving so much on the offense side that people forget to focus on defense. A good way is to have experience points for the king in defense. If you successfully defend a raid you get experience points, which is higher in case 2 crowns, which in turn is more than a single crown. Every attack on a kingdom gains a small exp. point for the king.
This way there will be motivation to stop dropping trophies by opening the base.
Of course the overall, exp. points can be increased to reach the next level.