IMHO, atheists tend to have real jobs.
Given that approximately 90% of Americans are still employed, I'd guess that
everybody tends to have "real jobs".
None of us is a preacher with 24/7 time to "convert". We do not have dedicated and paid "converters". This is a distinct disadvantage.
Why? Theists seem to have figured out a perfectly cromulent method of having real jobs
and supporting dedicated outreach workers in prisons. Charitable donations of surplus wealth (and even, sometimes, not-so-surplus wealth) figure prominently in this system. Maybe atheists should look into this?
So every voluntary effort is highly welcome and will probably make a difference.

Even volunteers need food and shelter. Theist outreach programs seem to be pretty successful at finding theists who care enough about the problem, and are committed enough to the solution, to donate resources and support those volunteers.
I don't think the problem is that atheists have "real jobs", since everybody has a "real job", including professional outreach workers. Rather, I think the problem is that atheists would rather spend their "real job" paychecks on something other than ministering to convicts--even though such ministrations would no doubt be "highly welcome and will probably make a difference".
As far as I can tell, atheists, by and large, don't
want to make a difference in this way. But maybe I'm wrong: Maybe atheists
do want to make a difference, but simply
can't. Not for lack of resources, obviously--they have the same access to the same resources as theists do, after all. Maybe the question that most needs answering is, does atheism actually have anything to offer criminals? Anything that might give hope, or peace, or respect for their community and its shared values?