1. Lion as a heraldic symbol has been used by many throughout history. Its popularity rise to the fact that the lion was seen as the king of beasts and it represented multiple things. In case of England it represented few things. The lion for England means dauntless courage, strength and England itself.
2. England had many enemies and it was one of the most powerful empires in the history of the world. So it makes sense for them to have chosen a lion as their symbol. They used so many of them in order to intimidate their enemies and that show that they are both courageous and powerful and they wanted everyone to know it.