Why Was General Grievous So Pathetic When He Fought Obi-Wan In Revenge Of The Sith?

Two reasons, and neither is that Grievous was overrated. Obi-Wan Kenobi was a master of Soresu, the defensive lightsaber form built to absorb aggression and punish the small mistakes it produces, which is the worst possible matchup for how Grievous fought. And by the time they met on Utapau the general was already broken goods, carrying a crushed chest and a cough it had made far worse. (Revenge of the Sith; Star Wars: Clone Wars, "Chapter 25")

He was not a fraud#

Start with what the general actually was, because the Utapau fight makes him look like a paper tiger and he was nothing of the sort.

He was born Qymaen jai Sheelal on Kalee, and he was a killer from childhood. In Legends he was an expert sniper with more than forty kills by the age of eight, and his own people came to regard him as something close to a demigod. He grew into a warlord with a genuinely bloody reputation, took the name Grievous, and led the Kaleesh against the Yam'rii, a technologically superior insectoid species invading from the neighbouring world of Huk. He did not just repel them. He drove them off Kalee and then carried the war onto their own worlds. (Unknown Soldier: The Story of General Grievous)

That is what caught the attention of Darth Sidious and Count Dooku. In Legends, Dooku, the banker San Hill and Poggle the Lesser had a bomb planted on his shuttle, and the crash that nearly killed him became the excuse to rebuild him as a cyborg. Dooku then trained him personally in lightsaber combat, and Grievous went on to build a career as a Jedi hunter, collecting the lightsabers of the ones he killed. He commanded the Separatist droid armies and became one of the Republic's primary targets for the length of the war. (Revenge of the Sith; Unknown Soldier: The Story of General Grievous; Labyrinth of Evil)

So the question is not why a mediocre fighter lost. It is why one of the most effective Jedi killers of the era went down in about a minute. (Revenge of the Sith)

Reason one: Soresu eats aggression#

Obi-Wan Kenobi mastered Form III, Soresu, and it is a genuinely awkward form to fight. (Obi-Wan Kenobi, "Part VI"; Star Wars Rebels, "Twin Suns")

Soresu was developed in answer to blaster supremacy, and its whole logic is defensive. It uses tight, controlled movements and a strict economy of action, on the reasoning that even a slight extra motion leaves an opening a blade or a bolt can slip through. A Soresu practitioner trains to block and parry, to hold off several opponents at once, and above all to keep a calm focus that produces the right defensive answer to whatever is coming. Stances are built to present the smallest possible target, which cuts down both the angles an attacker can use and the area the defender has to cover. (Knights of Fate)

Grievous fought the exact opposite way. His style was aggression stacked on aggression, four arms, four blades, spinning constantly and trying to overwhelm through sheer volume of attack. Against most opponents that works, because most opponents cannot survive the opening exchange. Against a Soresu master it just means a longer supply of small mistakes, and Kenobi was very good at reading them. (Revenge of the Sith; Knights of Fate)

Their rivalry ran for the entire Clone Wars and they duelled repeatedly without a decisive result, which tells you Grievous was never outclassed on skill alone. What changed on Utapau was that he was fighting a man who had by then seen his patterns dozens of times. (Revenge of the Sith)

Reason two: he was already wrecked#

The second reason is simpler. Grievous was badly injured before the fight ever started.

During the Battle of Coruscant, Grievous pulled off the boldest operation of his career. He led an assault on the Republic's capital world and took Supreme Chancellor Palpatine captive aboard his flagship, the Invisible Hand. It should have been the peak of the war for him. Instead the Chancellor was rescued by Kenobi and Anakin Skywalker, Dooku died, and Grievous fled into the Outer Rim. (Revenge of the Sith)

He also left with a crushed chest. In Legends, Mace Windu used the Force to crush the general's chest plate during the fighting on Coruscant, and the damage never came right. He had wheezed before, but now the cough got far worse, and it was not a character quirk. It was the sound of a fighter operating at a fraction of what he had been, wheezing his way through the last days of the war. (Star Wars: Clone Wars, "Chapter 25"; Revenge of the Sith)

The setup on Utapau#

Put both together and Utapau stops looking like an upset.

With Dooku dead, Grievous was left leading the Confederacy. He retreated to Utapau and, on Sidious' orders, packed the Separatist Council off to Mustafar, where they would shortly be murdered. Then Sidious quietly handed his location to the Republic, because by that point Grievous had served his purpose and a live general was an obstacle to Order 66. (Revenge of the Sith)

So the last fight of his life was a damaged, betrayed commander with a ruined chest, throwing the same aggressive style he had always used at the one Jedi in the galaxy best equipped to wait it out. Kenobi killed him, the war ended days later, and the Confederacy collapsed behind him. (Revenge of the Sith)