Because a) the difference between a L1 and L6 is already astronomical. If they added a L15, they would so far outshine the lower players it'd be pointless playing together.
and b) because when they added L6, they needed to add a new difficulty mode to still keep things challenging for the top-end players. If they brought out a load of new perk levels with new advantages, even HoE would become too easy.
and c) because only 1.7% of players have got Mr Perky. If you add in more levels, the percentage of players it would actually affect would be miniscule.