The main reason for this is due to Kenobi's long stint in hiding. Again, Kenobi has been in hiding for around ten years, slowly diminishing his connection to the Force. While the Force, and people's abilities with it, are hard to define in Star Wars, it remains the case that the Force acts as any other skill does.
Obi-Wan Was Too Exhausted To Use Force Speed To Reach Qui-Gon.
So, Vader let him go so that Obi-Wan could regain his full strength because what Vader really wanted was to prove that he was more powerful than his master ever was -- and offing "Ben Kenobi" would have been a massive letdown.
Originally Answered: Why didn't Obi-Wan use more force powers when he fought General Grievous? Simply because Force abilities and powers aren't his strengths. Obi-Wan's greatest strengths are his defensive lightsaber skills (he is the master of Form III) and his patience.
He also had to live with the fact he basically trained Darth Vader and watched him turn on the Jedi, killing countless people even after Order 66. Given the events of Obi-Wan Kenobi and the prequels, it is pretty clear that Obi-Wan's ultimate weakness is likely his PTSD.
Obi Wan is one of the most powerful Jedi to have ever lived. He is the best defensive duelist in galactic history according to Mace Windu and numerous factbooks. He had a low sensitivity to the force but mastered it to his fullest potential worshiping the force as opposed to bending it to his will.
The Force: Kenobi was extremely powerful, and had an extremely strong connection to the Force, and was one of the most powerful force users of his time. Though Kenobi did focus largely on Lightsaber combat, he was also very skilled and powerful in use of the Force.
Anakin and Obi-Wan confront General Grievous on the bridge of the Invisible Hand as it wages battle against Republic warships over Coruscant.
He left the scheming and negotiations down to other Separatists leaders and focused on upping his kill count and taking Jedi lightsabers as trophies. He hated the Jedi so much that Grievous even learned to wield lightsabers as a non-Force user, which was incredibly dangerous and usually foolish.
The first reason that Grievous and Anakin didn't fight in The Clone Wars was because the two of them couldn't have met due to a relatively unimportant line of dialogue in Star Wars: Revenge of the Sith.
Obi-Wan knew it was only a matter of time. So, he made the difficult decision to use his learnings and become one with the force, rather than let Vader decapitate him.
While Obi-Wan Kenobi was frail by the time of Star Wars: A New Hope, he was still stronger with the Force than Darth Vader. The duel between Obi-Wan Kenobi and Darth Vader in Star Wars: A New Hope looks dated by today's standards.
Darth Vader can not use force lighting, because of his severed arms. Robotic hands can't summon the force lighting. However Darth Vader can still use other forms of the force, such as force choke.
Qui-Gon literally said "it's too late" to Obi-Wan. Force healing can undo the damage, but not its effects. His body had already shut down at that point. Force Healing requires one to transfer one's own vitality to the wounded, so the more severe the wound the greater amount of life energy that needs to be transferred.
Theory 1: Leia Doesn't Remember Ben Is Obi-Wan
Throughout her encounters with Kenobi in the series, he refers to himself as Ben. The most straightforward explanation would be that Leia knows the person who rescued her is Ben, and never connected the dots that Ben is actually Obi-Wan.
Why does Obi-Wan Kenobi age like that? Because he's not real, and is a fictional character played by different people. You see, Obi-Wan Kenobi is played by Alec Guinness in the original trilogy. Guinness was 62 when he filmed A New Hope, and he died in 2000.
General Grievous's origin is revealed in the comic book "The Eyes of Revolution" from Star Wars: Visionaries. Grievous was originally Qymaen jai Sheelal, an organic reptilian humanoid native from the planet Kalee.
It was not until Chapter 25 (or Season 3 Episode 5 if you prefer) that Grievous had a showdown with Mace Windu. During the encounter, Windu used the Force to crush the chest plates of Grievous, severely injuring his lungs, and causing his now distinctive cough, which made its way into Revenge of the Sith.
Maul and Grievous met in single combat, while the two armies collided and suffered heavy losses. By the battle's end, Maul ordered his forces to retreat, while the Sith waited for him to find Mother Talzin.
Palpatine revealed himself to be a Sith Lord, Darth Sidious, and promised to train him in the ways of the dark side. Anakin eventually succumbed to this temptation, maiming former ally Mace Windu to save Sidious.
In the novel, Dooku remains silent when Sidious tells Anakin to kill him, because he finally realised, that he had been replaced. His master has found a new apprentice, younger, filled with more rawness and power to destroy the Jedi.
As Darth Vader, Skywalker was believed to have had roughly eight-tenths of the strength of the Emperor. Nevertheless, Vader wielded tremendous power and skill, enough to use the Force to overpower almost any opponent he could not defeat with his lightsaber.
#1 – Yoda. Yoda is widely considered to be the greatest Jedi of all time, and with good reason. He is a master of the Force, a skilled warrior, and has an unmatched wisdom that comes from centuries of experience. He is the embodiment of what it means to be a Jedi, and his legacy lives on long after his death.
This isn't to say that Mace Windu isn't powerful when it comes to the force. He's on par with Darth Maul and is more powerful than Qui-Gon Jinn. But, Obi-Wan is still the stronger Force user which makes him a powerful combatant.