We don't know for sure, but it seems possible that the sonnet form was chosen because of the sonnet's strict order and structure.
The events of the play will be chaotic. The words of the sonnet tell a story of potential violence and disarray.
Yet, these words are contained in a very orderly poetic form. The contrast of these two things adds complexity and depth to Shakespeare's prologue.
One other popular theory proposes that the sonnet form is chosen because Romeo and Juliet is a love story, and sonnets are associated with love. This may be the case.
However, it seems more likely that this theory is an interpretation that we can put on the prologue as we look back on the play. It may not have been a deliberate choice on Shakespeare's part.
We have no way to know for sure, as I said, but it is enjoyable to study and consider.