The key issue is not what is in the prayers, but that the prayers are repetitive. It seems to follow from this that prayers would not arise from a person's heart. Protestants generally believe pretty strongly that prayer should be spontaneous from a person's heart--not words that someone else wrote for them to read or recite.
In the Sermon on the Mount, Jesus taught us not to use "vain repetitions":
But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking. Matthew 6:7 KJV
The Psalmist also entreats us to pour out our hearts to God. In order to do this, rote prayers seems counterproductive.
Trust in him at all times, O people; pour out your heart before him; God is a refuge for us. Psalm 68:8 ESV
In fact, the Psalms are full of people pouring out their hearts to God. So, the model appears to be to pray from the heart rather than to repeat what others have written for us to pray. Of course, Protestants typically believe pretty strongly that we should read the Scriptures pretty regularly.