Sorry if I wrote that vaguely, he did _not_ syndicate. He was burnt out and it was either close up shop or syndicate, he chose to close up shop.
I may be off on this a little, as I did a little reading to verify. Waterson was opposed to having other people write and draw the Calvin & Hobbs cartoon for him. He noted that so many other cartoons had done this and their quality had plumetted.
From memory (because it was written in his last book, and I can't find it online) he said that when he couldn't make a new cartoon every day and be happy with the quality, he would be done.
Edit: I may be conflating syndication with licensing.... :P