The Emperor of the Galaxy was placed in suspended animation shortly before his death, meaning that he has now lived for millions of years and all his potential heirs are long dead. This has caused all power in the galaxy to move down a rung. This is the reason why the Presidents of the Galaxy, such as Zaphod Beeblebrox and Yooden Vranx were really doing nothing except cover for the man really controlling things, The Ruler of the Universe.