The reason they probably didn't drop them right away was that Apple initially believed Nvidia, that the problem only affected a very limited number of laptops.
But now, the installed base of hundreds of thousands of macbook pros with the 8600m have been aging, and Apple is probably getting a huge and growing wave of users who are having the problem (since it reportedly takes some time to develop in most cases).
Apple would not have known just how extensive this problem was until time passed and tens of thousands of macbook pros showed up with the problem.
Therefore, it makes perfect sense that Apple would have not decided until now to drop Nvidia.
No, not quite. The answer is far simpler than that: Time. It takes time to make changes. You think a company the size of Apple can dream up a new piece of hardware and change-over in a couple of weeks? It takes times to iron out the legals and then finally change the production line. Anyone who thinks it's as easy as ordering a few thousands crates of new components and then slapping them in is deluded or pleasantly naive... or a pleasant naive