I could probably argue the writer thing, there's a lot of bs from V3 before the ghostwriters too, but keeping it in story I would say it's an institution thing.
Esplin's role in the building of the Yeerk empire in HBC and TAC can't be understated. This was a literal brand new society for the Yeerks, he really was part of its foundations. He learned about the Andalites and founds ways to exploit them during the Hork-Bajir war. Edriss may have started conquest, but he was the one who found out a class five species exists. And yes, taking an Andalite, especially a war-prince, especially the one who showed the darkest side of the Andalites, was a huge move for the Empire. This guy was making huge strides when the Empire was in it's infancy. And so, he got institutionalized.
Most leaders do great things, and then either die or give up their position. Those who stay on forever don't do great things forever, in fact they often get paranoid or ignorant in their arrogance. This guy was a least a sub-visser for 30 years. He was almost always in a position of power, and made mistakes. And a lot of them. But he'd shown he could do some of the most important things that the Empire needed to succeed. It's like if Lincoln came back from the dead, get him up to speed he could probably be a decent president, and a lot of people would want him just because of what he did back when, but keep him in office another 30 years and he's bound to do some stupid stuff.
That, and the Yeerk Empire, maybe in part because of it's young, seemed bereft of leaders. They kept Edriss after all her b.s. The sub-vissers were by an large little more than sergeants in most cases. The Empire was still learning how to wage galactic war, they didn't know all the signs of a bad, even a terrible leader. Plus the council liked the way he toadied to them; he knew Yeerk psychology and used it. A thousand Yeerks could have tried to assassinate him, but for all we know never did, because cruel, tyrannical and merciless were marks of a capable, if not good, leader in a society that's first steps were kill, conquer, enslave.