I wonder if there just stopped being interesting questions to answer. I remember him saying somewhere that he got a lot of questions that read as if the asker was trying to create a specific answer.
[1] https://www.nytimes.com/column/good-question-randall-munroe-...
It's pretty telling that What-If has 573 subscribers on Inoreader (the RSS feed reader), but Good Question only has 3!