In Chapter 12, we learn that it is because hardly any of the congregation can read: Zeebo (Calpurnia's son) leads the hymns, singing each line and having everyone repeat it back to him.
The other reason, of course, is simply because the church is so poor: and Lee undoubtedly presents it as more spiritual because of its lack of material wealth. From the book:
First Purchase was unceiled and unpainted within. Along its walls unlighted kerosene lamps hung on brass brackets; pine benches served as pews. Behind the rough oak pulpit a faded pink silk banner proclaimed ‘God is love’, the church's only decoration except a rotogravure print of Hunt’s The Light of the World. There was no sign of piano, organ, hymn-books, church programmes-the familiar ecclesiastical impedimenta we saw every Sunday.