Apple Maps currently offers cycling directions in parts of three countries:

Of the five areas in the U.S. where Apple currently offers cycling directions, all five are among the top ten U.S. cities in terms of the total number of bicycle commuters:

This suggests that Apple may be prioritizing cities with high numbers of bicycle commuters.

If so, then Chicago, Seattle, Washington (DC), Philadelphia, and Minneapolis are the most likely U.S. cities to receive cycling directions next.

UPDATE | April 27, 2021

In late April 2021, Apple added cycling directions for Seattle and all areas within California.