Because it must be a voluntary choice instead of forced. A free trade of reproductive capability in return for a reduced sentence. To avoid overly targetting certain groups, the offer could be standardised in some way.

Ultimately I believe society would benefit by breaking such toxic cycles.

The problem with incentivizing sterilization is that you’re essentially abusing power dynamics to coerce vulnerable people into doing something they would otherwise not choose.

You also need to consider that a staggering number of men in prison in the USA for decades were jailed over cannabis, which is now legal in more states all the time. These are not people who we need to sterilize.

Any situation where an authority incentivizes a marginalized individual to be sterilized, there’s a serious problem.

I'm not sure I believe incarcerated people can make free choices. And I would assume that legislators would push towards harsher and harsher sentences to incentivize prisoners to "choose" sterilization if they ever want to see daylight again.

An incarceration system, if it has any hope of working, must treat prisoners as people who can become valuable members of society in the future despite their past wrongdoing. When you decide that prisoners are a blight on society and that no rehabilitation is possible then prisoners turn into some kind of underclass that is to be disposed of, either directly or indirectly. That's real dark.