I am not sure ‘earns’ is the right word, but it is, as Governor General Quentin Bryce noted in her congratulations to John Howard, ‘a rare and singular honour for his service to Australia.’
It is rare in that only 24 persons can be members at any one time (other members include Baroness Thatcher, Prince Charles, and Tom Stoppard), and singular in that he is the only Australian politician to whom this honour has ever been granted. Other Australians admitted to the Order of Merit include Howard Florey, Sidney Nolan and Joan Sutherland.
If you are not sure why he deserved to be honoured in this way, why not buy his autobiography?
The Kindle edition is only $15.25. It is a great read. Not only is Howard a good writer, but he is consistently fair to both colleagues and political opponents.