Solving a few more murders in Chicago probably won’t change much in the city. Working with uncooperative witnesses isn’t a good use of time.
Solving this hoax probably will make a change. The goal of the hoax was to make minorities afraid of white men and especially afraid of Trump supporters. That’s 10s or 100s of thousands of people walking around Chicago feeling unsafe when they see a white guy if the hoax succeeds (not to mention all the people in the rest of the US). That’s a lot of tension that can be diffused by showing this to be a hoax IF the media covers the hoax as much as they covered the attack.
The hoax doesn’t just have a negative impact on Chicago society, but could very well change elections. Elections may not have been a direct goal of the hoaxers, but moving voters away from republicans was. And the media that ran with it certainly considered the impact it could have in 2020. The killing of a 1 year old is important, but a hoax that the media can go anti-republican 24/7 for a few days/weeks then come back to in the next election is much more significant. And if a thorough investigation stops future hoaxes it was well worth the extra manpower.
Why would a witness cooperate if the prosecutors won’t prosecute?