The particular Muʿtazilite reading of tawḥīd regards the Qur'an as created. Its stated reasoning is that, if the Qur'an were also eternal, there would be more than one eternal reality.
Eternity belongs to Allah, while His speech is His attribute, not another independent divine being. Calling the divine speech created undermines the proper affirmation of Allah's attributes.
Allah's essence and attributes are not separate deities, and affirming divine attributes does not compromise His oneness.