Here's my little bit of analysis for why this is a chickenshit decision. There's 4 main political groups that Chicago politicians want to keep in the good graces of:

1. CPD. Despite media puffery about Snelling being a witness in trials of cops, the FOP really likes him and is happy that Johnson appointed him. Like, really likes him. Catanzara is happy about it, and right-wing media is struggling to find anything bad to say about Johnson beyond general dissatisfaction with the state of the city. And so far he has worn kid gloves to deal with the police. WBEZ "How Chicago Mayor Brandon Johnson has prioritized his first 100 days":

He has been calculated in his comments on police and has surprised some people in the relationships he has built with the Police Department thus far.

Johnson’s choice in Snelling, for instance, has gotten a nod of approval by tough-on-crime politicians, including his former mayoral challenger Paul Vallas, who said Snelling “would certainly have been on my shortlist as a potential superintendent.”

Sposato praised Johnson’s approach with officers.

“Friends of mine that are high-ranking police officers, these meetings they have with the mayor and the interim superintendent, are just like a pleasure to go to,” Sposato said. “In the past, nobody wanted to go, they were worried about getting demoted. Now, it’s just like, ‘Hey, what do you got going on? Let’s talk about stuff.’ And it’s a pleasure to go to.”

While Johnson’s comments in early August scolding a reporter for referring to the mass gathering as a “mob action” led to criticism from the right, Johnson also had praise for the police, who made 40 arrests, as a result of the incident: “The level of sensitivity and patience that our officers expressed, I’m appreciative of that. That is constitutional. That is a system of care.”

A system of care is cops caging 40 people?

Cops will never be truly satisfied with a Democrat mayor unless he's literally a cop, and they still give Eric Adams in NY problems sometimes. A smart politician will will understand that at some point you get diminishing returns on trying to appease the FOP and CPD at in general. While the rank-and-file has been grumbling about having the migrants sleeping in the station, because it makes manning the front desk more difficult, I think that he gave leadership such a boon by picking a top cop from CPD ranks that it is not urgent to get the migrants into tents just to keep CPD happy.

2. CTU. Obviously BJ is a CTU man, they're happy with him, and they have no interest in the fate of the migrants anyway.

3. Local businesses, bars and restaurants, hospitality industry, etc. These guys don't have any interest in the migrants. Maybe they want them as part of the reserve army of labor / to drive down the price of "unskilled" labor.

4. Local real estate, landlords, and developers. If the city was to buy up a bunch of buildings, condos, or even just rent market-rate apartments for the migrants, these guys will directly benefit. The city might buy or rent from them, and demand for another 2000+ units would allow them to charge more money for the housing stock the landlords already own and raise the value of whatever the developers are building.

I think that having the extremely visible migrant crisis, where these new homeless people are concentrated and visible every time you pass a police station or turn on the news, is a golden opportunity for Johnson. Because the plight of the migrants is basically the same plight as the rest of Chicago's homeless population. If he gets migrants out of the police stations and into housing, people will notice and it will be good for his approval rating. By setting up a good, sustainable solution for that, he could also spend money to fix Chicago's homeless problem without public ire (because nobody gives a shit about homeless people other than the migrants). Shelter availability is terrible; if you manage to get a bed you have 2 months to get a job before you're kicked out, and the wait for permanent housing is years long. People have been criticizing him for doing basically nothing, and knocking out two birds with one stone with a robust public housing program would solve that.

Instead, he's picking a cheap band-aid fix to get media and CPD rank-and-file off his back. Every Chicago mayor is the same.