"Wizard-Kings always fall" goes the ancient saying. A wizard who seizes secular power must give up some measure of his magical progress. His hours and days become consumed with the demands of rulership. Meanwhile, his rivals and challengers are free to redouble their research, often spending every waking hour on their studies. Eventually, one such rival will unlock enough mystical secrets, gather enough arcane knowledge, and unleash enough eldritch power to eclipse the Wizard-King in magical strength and topple him in a clash of magics, seize the crown, and fall into the same trap.
Some mystics who would walk the path of rulership seek to avoid such a fate by dividing their time, only to lose ground in both the political and magical arenas. Only so much of the work of ruling can be delegated, even if enough trustworthy and talented people could be found to take on the burden. Only so much time can be pried from other responsibilities for the research that is a wizard's passion, especially compared with those free of such burdens. Political rivals will take advantage of the Wizard-King's divided attention to gain influence. Rival wizards will focus on their researches and inevitably close the gap in magical power.
It is commonly accepted that the best place for those who follow the mystic arts is to advise and assist those who already rule. This role is relatively undemanding and the time it requires can be spared more easily. While direct rule offers more political power, the influence of a trusted advisor is considerable.
But there are always those who try to use their mystic might as a stepping stone to political power. They think themselves too cunning to fall into the same traps that ensnared previous Wizard-Kings. A few have actually succeeded.
The Wizard-King Ballantyne outlawed magic throughout the realms he conquered to prevent the raise of rival wizards. One such rival escaped his notice. She sought out allies among those without political ties in Ballantyne's realms. Outland barbarians raided those who worked to bring forth the bounty of the land and wealth of its markets. Bards labeled nobles close to the Wizard-King as despots, justifying their later assassinations. A few bold adventuring parties assaulted Ballantyne directly – they failed to kill him, but injury and paranoia hampered his activities. Meanwhile, the rival wizard used her own magics to cloud the Wizard-King's scrying and counter his spells. In the end, the Wizard-King and his base of support were so worn down that his realms welcomed a challenger – a callow youth armored against Ballantyne's magics and wielding the sword fated to pierce the Wizard-King's heart. That youth was elevated to the throne by those who thought him easy to manipulate, but they were thwarted by the appearance of his chief advisor – a woman of great beauty and insight.
There are the tales of the Lich-King Koschei, who began his rule as a mortal man, but used the rituals of Lichdom to strip the flesh from his form and gain an unholy form of immortality. Without the need for sustenance or sleep, he could rule by day and study by night, making every moment of his undead existence count. As the years wore on his studies unlocked knowledge of the outer planes. His interest in the material world declined. He was removed from power by an alliance of his treacherous apprentices and rebellious courtiers, but tales hint at his survival and eventual return.
Related in principle to the Lich-King was the Necromancer Lord. His source of power was not reality-bending spells, but his endless army of undead. His rivals, both political and magical, were simply overwhelmed by the cold, unliving hands of his followers. Shortly afterwards, those unfortunates found their minds imprisoned within their own animate corpses. Enslaved by necromancy, his former rivals knelt at his feet and offered their undying support. Only his own death ended his rule – as he found himself dying of age, he attempted the rituals of Lichdom, but his ailing body caused his spells to falter. His corpse was consumed by his own army of undead.
(This was some background for a D&D campaign that never got off the ground. I found it, dusted it off, and gave it a quick edit before posting it here.)