When Israel was exiled, in the days of Nebuchadnezzar the Wicked, they intermingled with Persia and Greece and other nations, and they had children in foreign lands. And these children spoke hybrid languages and everyone spoke a different dialect. And when they had to speak, they could not express themselves clearly in one language, but instead some hodge-podge, as it says, “And their children spoke half in Ashdodite, and could not recognize Jewish speech, and spoke according to the language of each nation they were in.” (Nechemiah 13:24) And because of this, when one of them would pray, they would cut short their prayers for their needs, or their attempts to praise god in the Holy tongue. And it got to a point where Hebrew itself was getting mixed up with other languages. When Ezra and his court saw this – they arose and instituted the eighteen blessings in their order.
The first three were Praising God, and the final three were Thanks. And the middle were Requests for everything - major categories that covered all human needs, individual and communal.
This is so that they would be ready on everyone’s tongue, and could be learned quickly, and so the prayers of those who spoke poorly could still be as complete as the prayers of the most articulate people. And so they instituted all the blessings and the prayers in a fixed order, so that they would be on the tongues of all of Israel, and so that every blessing would be laid out clearly for those who spoke poorly.