Catholics are indeed involved in social justice. We practially own the entire field in the pro-life movement, to note the most obvious example. This is not intended to slight the many non-Catholics who are also working for the right to life, which is truly the greatest social justice issue today.
I donāt know why those particular Catholic parishes in your area would decline to participate in the particular ministry you are associated with. That is something you would have to ask them. But I could suggest some possible reasons.
The program you describe sounds similar to one we have here in McHenry County, called PADS (Public Action to Deliver Shelter). During the cold months, seven churches take turns one night per week, to provide overnight shelter, supper, breakfast, a bag lunch, counseling, financial assistance, social services, physical and mental health services, transportation and other services for homeless persons. You might be interested in knowing that in our county two of the host churches are Catholic, two are Lutheran, one is Episcopalian, one is Methodist, and one is Congregational Unitarian. Other Catholic parishes, other non-Catholic churches, Synagogues and other non-Christian religious communities, civic organizations, and other organizations that do nor act as host sites still support this effort by donating money, food or other needed supplies, and parishioners and members from these churches and organizations all volunteer to cook and provide other assistance at the host sites.
This system works well, and really would not be better with more host sites (unless the number of homeless persons being served increased such that more space was necessary), because now the homeless clients know where to come every day of the week. Sunday evening is always Bethany Lutheran, Monday is always SS Peter and Paul (Catholic), Tuesday is always St Patrick (Catholic), etc. Scheduling more churches into the mix could cause confusion with homeless people going to the wrong site on the wrong days, which could be tragically dangerous in the winter.
The question could be asked why these churches volunteered to be host sites while others didnāt. The reasons could be many. Some donāt have enough space suitable to bed down upwards of a hundred people, segregated by men and women, with some separate spaces for parents with children. Others donāt have kitchen facilities adequate to feed this many people. Some are legally constrained by fire marshall occupancy codes, or, especially if there are also parochial schools on the premesis, laws or regulations forbidding persons who have not had criminal background checks from being on school grounds or in school buildings when children may be present (as children may in the evenings for sports practices or other extracurricular activities.) As has been noted before, many churches that do not act as host sites still support this program at those churches that do host it.
However, there are also some interchurch efforts that Catholic Churches in our area do not participate in, because some aspects of those interchurch efforts, which might otherwise be laudable, are in serious conflict with the Catholic Church. One example of this is an event called CROP walk, which is an event intended to raise money to feed the hungry, both locally and internationally, but which our Bishop has directed Catholic parishes, schools and other organizations in our diocese not to participate in or financially support because because CROP walk supports abortion and contraception:
osv.com/periodicals/show-article.asp?pid=751
Catholics in our Diocese do participate in
Operation Rice Bowl and other efforts to feed the hungry.
Another possible reason I could suggest that Catholic Parishes might not be involved in specific interchurch efforts is that some interchurch groups ask their members to affirm a statement of faith which may contain elements not compatible with Catholicism, and refuse to admit members who will not do so. There is, for example, a christian homeschooling group that I know of that has no Catholic families because member families have to affirm sola scriptura and sola fides.
Finally, Catholic parishes are not involved in some interchurch social justice efforts because some non-catholic churches just do not want to work with Catholics. I am aware of very few of these groups, but still it is their privilege to associate with whomever they wish.