Why are hymnals not used in the First Purchase A.M.E. Zion Church in To Kill a Mockingbird?

Hymnals are not used at this church for a couple of
reasons, although I do not think that Lee actually comes out and says
why.

First of all, having hymn books for everyone would
cost a lot of money.  You can see from the description of the church that they don't
have much money.  Even the one hymn book the do have is all beat up, for
example.

The second reason is that you can assume that most
of the people who attend the church do not know how to read anyway.  So having books
would not help.

So instead of all having books, the music
superintendent "lines out" the hymns and they all sing.