Some readers here may know that I'm a book editor who also writes indexes. I think the reason for omitting an index was simply time. The publisher wanted speed. Including an index would have added at least two weeks to the schedule.
Nonetheless, two smarty pantses have provided their own indexes. Here's a sample from Slate:
________autumn bouquet of, 1
________robin's egg sky of, 2
________superiority to Lower 48 of, 1-413
Who knew that indexing could be fun?
h/t Andrew Sullivan.