I can't pretend to know why they had that symbol but here's my personal opinion,first of all the appearance of a male lion with it's huge mane,kind of resembles a king with a crown of some sort and just like kings sat on their golden chairs for hours doing nothing,that's pretty much a male lion sitting on an old termite hill for hours on end,doing nothing but watching the horizon and hoping some herbivores tiptoe along the horizon.
On the other hand a king's life in the past like we see it in movies today,probably involved swords and constant civil wars to protect a monarchy as well as territories and just like lion society like i have mentioned so many times,is brutal and a male lion has that responsibility to fight off enemies to protect a pride and it's territory and if you can't see the comparison,I would like to know any other wild animal that lives like lions.Obviously male elephants don't sit there waiting to be served and am sure a king does and so does a pride male.