The 'Lighter' Side Of The Force
Windu thought that Obi-Wan was the answer for Grievous. Grievous was not a true force user, so Obi Wan would be able to concentrate almost entirely upon physical issues. Soresu was about "never letting anything through". So long as Obi-Wan kept his opponent at a specific distance (where he would be less likely to end up in blade lock), he would be able to arrive at any opening in his defense well-before the attack was executed. This, along with Grievous inability to bring other force abilities to bear (Obi-Wan wiped out a bunch of his bodyguards with a simple gesture), meant that Grievous was destined for failure. This was really a matter of Windu's judgement. He had seen so many Jedi killed by Grievous, and had failed to achieve victory himself. Grievous presented a unique challenge for which Obi-Wan provided the perfect answer.