When writing the Reformed Church Order, why did our forefathers limit the songs to be sung in the church worship service to those listed in Article 69?
It keeps the church liturgy focused upon God's words, not man's. Even when the hymn is doctrinally sound, the congregation is singing man's words and not God's, when worshipping Him.
It prevents unscriptural phrases, emphases, or ideas from being confessed in the worship service. God's Word is infallible; the best of man's words is not. Certain hymns are more man-centered than God-centered, dwelling more upon man's fallible experiences and feelings than on God's infallible truths.
In accordance with their desire to keep the church worship service focused on God's Word, our Reformed forefathers wrote the following article in the Church Order of Dordt (1618-1619):
In the churches only the 150 Psalms of David, the Ten Commandments, the Lord's Prayer, the Twelve Articles of Faith, the Song of Mary, that of Zacharias, and that of Simeon shall be sung. It is left to the individual churches whether or not to use the hymn "Oh God! who art our Father." All other hymns are to be excluded from the churches, and in those places where some have already been introduced they are to be removed by the most suitable means.
9. The doxology - The word "doxology" is derived from two Greek words, "doxa" meaning "praise or glory," and "lego" meaning "to speak." A doxology is a short song or words of praise in a worship service which ascribes glory to God. Many portions of Scripture are "doxologies" and can be used as such in worship.
The three most common doxologies in Reformed churches, which use The Psalter, have been the following.
Praise God, from whom all blessings flow,
Praise Him all creatures here below,
Praise Him above, ye heavenly host,
Praise Father, Son, and Holy Ghost.