I can only assume.. to some of you it might seem crazy but I have a good friend who is really top notch concerning stuff like that (Matteney wise too).
We basically think that he doesn’t want to ruin what he had with Courteney on FRIENDS.
You have to know that when he denied the offer, he said that he thought it “might be too much after Monica and Chandler”, so he thinks that people will still see Mondler when watching him acting with Courteney again.
To him, what they had (on and off screen) was perfect - Mondler got married and now ‘live happily ever after’ and he doesn’t want to destroy that image by getting on CT, knowing it will end up in a breakup. Because, let’s face it, he would have played her love interest so it would’ve only been for one/a couple of episodes. I think that thought creeped him out, he wants to preserve the perfect ending with Courteney.