Because at some point, they realized they weren't actually going to be able to catch the person who did it.

If they didn't arrest someone, they would have invited copycats.

They can't let the public even consider that vigilante justice is possible. At all times, they must appear competent.

Based on what they were saying during the shooting investigation, they were truly stuck.

Whoever did it was Law Abiding Citizen-level meticulous. I don't want to speculate too much, but one thing is clear:

There's no chance in hell that, if they caught the real shooter, they would have found incriminating evidence on him, not unless he wanted them to for some reason.

They arrested someone to control the narrative.

There's a truth we're not allowed to acknowledge: violence has often been used throughout history as a response to disenfranchisement and injustice.

Many people in the modern world have been conditioned to believe that only the state can decide when violence is justified. In that sense, we've domesticated ourselves. Yet every individual remains capable of resistance, and powerful institutions are not infallible.

They are comfortable in their control, and the most dangerous thing this shooting did was remind people that we're not powerless.

We just live in a state of learned helplessness.