While it isn't explicitly mentioned in the films, Obi-Wan is known amongst the Jedi as the master of Soresu (Form III), the most defensive lightsaber form. Some speculate - including me - that Obi-Wan and Anakin completely retrain their approach to lightsaber combat after their humiliating defeat to Count Dooku in Attack of the Clones. Obi-Wan becomes the master of Soresu - defensive fighting. In contrast, Anakin trains himself in Djem So (Form V), a much more offensive and aggressive fighting style (which still retains some defensive aspects from Soresu). As Kenobi says when Count Dooku confronts them at the start of Revenge of the Sith, "This time we'll do it together," implying that he'll be doing the defensive work and Anakin the offensive. By doing this, they can keep up with Dooku's superior swordsmanship and work together to defeat him. In theory. Of course, it doesn't quite go to plan, as Dooku focuses on separating them. He then dispatches with Obi-Wan, leaving him to take on Skywalker alone. Anakin eventually defeats him, but not through his lightsaber form. Rather, he briefly gives in to his anger and hatred, using the dark side to overpower the Count. Because Obi-Wan is the master of Soresu, it's the perfect defense against Grievous' four-fold attacks. Grievous relies on intimidation and brute Force to get the better of his opponents, and so the most defensive lightsaber form is needed to counter him. Finally, the Jedi knights are spread thinly across the galaxy. Yoda is needed on Kashyyyk, and many of the other Council members are fighting across various systems. Only a few Jedi Masters are actually at the Jedi Temple on Coruscant at this time, and some need to stay to defend it, of course. It seems that the Jedi are spread so thinly that they can only afford to send one person to confront Grievous - and there is no better choice than Obi-Wan.