That was because there were crowds of "people milling in front of the church". So, it might have been difficult for him to move through this crowd to find a parking space.
Explanation:
In the book Roll of Thunder, Hear My Cry, we learn of the scene where Uncle Hammer takes his sister and nephews to church on a Sunday. The likely reason why Uncle Hammer packed in the school grounds instead of driving up to the churchyard was because of the cluster of churchgoers going through the churchyard. He might have found it difficult to navigate through the crowds of people to find a parking space.
As they drove into the school grounds, the other churchgoers who were clustered in front of the church were wondering who the people were in the Packard. It was not until Uncle Hammer came down from the car that they were able to identify him.
