Why Was General Grievous So Weak In Revenge Of The Sith?

The out-of-universe answer is that the character was not fully developed when Revenge of the Sith was made, after the Clone Wars micro-series had already turned the cyborg into something close to unkillable. The in-universe fix is that Mace Windu crushed Grievous's chest with the Force during the Battle of Coruscant, severely weakening him and producing the wheezing, cowardly general we meet in the film. (Star Wars: Clone Wars; Clone Wars, Chapter 25; Revenge of the Sith)

He was genuinely terrifying before the film#

It is worth establishing how far the drop is. General Grievous was one of the most proficient Jedi killers in known galactic history, capable of holding off several Jedi Masters at once. His reputation was not marketing. (Star Wars: Clone Wars; Revenge of the Sith)

Early in the war he tested the IG-100 MagnaGuards at the battle on the fourth moon of Parein II and utterly crushed the Republic force there, whose Jedi General Sannen was stabbed through the throat by an electrostaff. The MagnaGuards became as feared as their master, and he was rarely seen without at least two of them forming a shield around him. (The New Essential Guide to Droids; Revenge of the Sith)

He was also responsible for the hour-long orbital bombardment of Humbarine, a heavily populated and urbanised world and one of the founding members of the Galactic Republic. He forced a public surrender out of Hoolidan Keggle on a galaxy-wide HoloNet transmission. (Star Wars: Revenge of the Sith: Incredible Cross-Sections)

Against Obi-Wan Kenobi specifically, he had already won. During one campaign he boarded Kenobi's command ship without losing a single capital ship, tore through the clone ranks, crushed a trooper's neck under his foot, then duelled Kenobi and overpowered him, kicking the Jedi back against a gunship. Kenobi only escaped by abandoning his own ship and rigging it to blow. (The Clone Wars, "Bound for Rescue")

That is the Grievous who shows up on Utapau and gets taken apart. (Revenge of the Sith)

What happened at the Battle of Coruscant#

The Legends explanation picks up here. During the raid, Grievous flew down to the planet himself, landed in the Executive District and fought through layers of Republic security, clone troopers and Jedi protectors to reach Palpatine. He killed Jedi Master Roron Corobb and claimed his lightsaber. He incapacitated Shaak Ti with electrified cables and took her weapon too. (Star Wars: Clone Wars)

Yoda and Windu were pinned down by droid fighters strafing the ground, which bought Grievous the window he needed. But not before Windu got hold of him. (Star Wars: Clone Wars)

Windu crushed Grievous's chest with the Force as the general escaped with his prize. The scene is Chapter 25 of the Star Wars: Clone Wars micro-series, and the 2005 novel Labyrinth of Evil follows the same events, with an injured Grievous carrying the captured Chancellor up to the Invisible Hand. (Clone Wars, Chapter 25; Labyrinth of Evil)

For a cyborg whose organic remains are sealed inside an armoured shell, that is not a bruise. That is structural. (Star Wars: Clone Wars; Revenge of the Sith)

Why Kenobi beat him#

Once you know about the chest, the Utapau duel reads differently. Kenobi does not out-fight him. He goes for the damage. (Revenge of the Sith)

As they fought hand-to-hand near the Soulless One, Kenobi managed to rip open Grievous's previously damaged chest plates, exposing the synthskin gut-sack holding his remaining organs. He grabbed the loose corner plates and pulled with all his strength until the plate came away. (Revenge of the Sith)

Grievous panicked. He seized Kenobi and hurled him off the platform, then, deprived of most of his protection and realising the odds had turned, he adopted his usual tactic of fleeing a battle that had gone against him. He went straight for his waiting wheel bike, scuttling away on six legs. (Revenge of the Sith)

That is the whole film performance in miniature. He is not weak so much as compromised and aware of it. (Revenge of the Sith)

Canon rewrote the cough#

Disney canon does not use the Windu explanation. There, Grievous's frailty is original equipment. (Star Wars: The Complete Visual Dictionary; Revenge of the Sith)

The technology used for his conversion into a cyborg was not perfect. His heart and lungs were weakened, and those lungs were irritated by their implants, giving him a persistent cough for the rest of his life and eventually difficulty breathing. His already compromised lungs meant his speech occasionally degraded into a rasping, random coughing fit. (Star Wars: The Complete Visual Dictionary; Revenge of the Sith)

The trade-off was worth it to him in other ways. He does not need to breathe, eat or drink, he can survive in vacuum and underwater, and he is immune to poisons and toxins. (Rise of the Separatists)

The two versions also clash on timing. Windu crushes his chest in Chapter 25 of the micro-series, but Grievous already coughs in episodes of the 2008 series set before those events. Whether the cough is a wound or a birth defect depends on which continuity you are reading. (Clone Wars, Chapter 25; The Clone Wars, "Grievous Intrigue")

So which is it#

If you want a single in-universe explanation that makes the film consistent with the Clone Wars cartoon, Windu is your answer, and it is well supported in Legends material. (Star Wars: Clone Wars; Labyrinth of Evil)

If you are reading current canon, the honest answer is that Grievous was never quite the invincible machine the micro-series made him look. Dooku trained him with a lightsaber and he collected trophies from Jedi he killed, but the body underneath was always failing, and every duel was against a clock. On Utapau, Kenobi found the seam. (The Clone Wars, "Grievous Intrigue"; Revenge of the Sith)