There’s a funny explanation at the bottom:

It turns out that only 9 of the first 275 checks that I've sent out since the beginning of 2006 have actually been cashed. The others have apparently been cached. So this change in policy will probably not affect too many people. On the other hand, I don't like to renege on promises, so I shall do my best to find a suitable way to send money to anyone who really prefers legal tender.