What does homogeneous have to do with prison rehabilitation? I'm not attacking you, genuinely curious.
If I think "it's one of those people from the other side of the tracks" it won't feel like a person with whom I could have a close connection. I might not care as much.
Not saying it's a good thing, though.
I'd say you would care too much! (abstract/general 'you', not @CalRobert). The problem any sort of reform faces is antipathy, not apathy.
Or you could say that it's an appeal to numerous studies (psychological and sociological), most of human history, and prudence...
Often, you'll hear people say something like, "Norway did X and it works, so we should do X in the U.S." These are typically fairly naive claims. Regardless of the normative dimension of the claim (whether X is worth doing in the U.S. in the first place), there is a non-trivial amount of complexity that is swept under the rug. For instance, NYC has twice the population of Norway. The U.S. really is significantly more demographically diverse than Norway (or, if this claim isn't good, then we can just say they are equally diverse, but at the scale of the U.S. this diversity increases complexity a non-trivial amount). The U.S. has a more fragmented legal system. Etc. Etc. Etc.
If a Martian were asked which nation would be easier to implement new policies in, maintain those policies, and have them bring about the desired end, which do you think they'd choose? I wager they would immediately choose Norway.
We also have a much more diverse population than Norway, so if anything, US should be looking towards us for inspiration.
Instead of using race as the example, let's use language.
You have three people. Two have spoken English their entire lives. The third grew up speaking French and learned English later.
They need to do something really hard to get right, something easy to mess up. Mistakes come at a high cost.
When the two English speakers work together, things work. They go smoothly. They can manage to overcome challenges and meet goals.
When they try to work with the French speaker, you see a lot more problems. It's frustrating. He does things they didn't expect. It is easy to feel like he's intentionally screwing things up.
Homogeneous demographic groups have an easier time all being on the same page. They spend less time and effort trying to bridge differences in experience, attitude, etc.
You get ugly terms like Mansplaining and Whitesplaining because "privileged" people -- people for whom life has tended to work -- often feel that their life works because they were taught the right information and if people who are having a hard time just had the same info, their life would also work. And then the person getting the info gets mad because they feel like they are being treated like they are stupid and not trying rather than like their life actually works differently because they are different and the world treats them differently.
It's all white people.
Sorry to be so glib, but I don't think I've ever seen "homogenous" used by someone who wasn't engaged in at least borderline racist dog whistling. The entire idea of it is based on the notion that people cannot successfully mix together, and therefore should not try.