You assume that the prosecutor is interested in who actually did it. In reality they are interested in convicting people for crimes (there is a difference).
Assaults are relatively minor, so if the prosecutor gets a witness identification he probably thinks it is conviction-worthy. I highly doubt he/she has spent even an hour trying to verify whether it was actually Trevor after the witness identification.
Sure, they would like to get the right person, but in the case of young urban minorities, they usually take the "guilty until proven innocent" approach. It is not surprising at all that they are going after him despite someone continuing the same sort of crimes. To the authorities, it is irrelevant if they can get a conviction for one of the assaults.
If you don't believe me, spend some time at a North Minneapolis community center.