Only one of the stamps - the 3c - features the King himself. In this case he is shown with his wife Queen Mary. Three of the other stamps also feature a member of the Royal Family. The 5c features the future Edward Vlll. I suppose this is understandable. After all, the king was nearly 70 and the people might want to see a picture of the heir apparent.
However, it seems slightly strange that the 2c should feature a picture of the Duke of York - the future George Vl. After all, at the time he was only second in line to the throne. And why would Princess Elizabeth feature on a stamp? After all, she was only the King's grand-daughter.
Presumably the reason why they were featured is that it was pretty obvious that Edward Vlll was not going to be king for very long. After all, he was 40 and was still unmarried. Although the British press managed to cover up the fact that he was going out with the twice-divorced Wallace Simpson, the American press covered details of their courtship. Many Canadians would have realised that the crown would soon be going to George and then to Elizabeth.
This is my theory. If anyone has any thoughts on the subject, let me know.
see also - How stamps helped George V survive World War One