There was also a migration of sorts from Louisiana to points east post-Katrina - and Louisana is historically Catholic, so that seemingly odd concentration doesn’t really surprise me much.
This map isn’t a real eye opener to me. And I don’t believe that other religions keep any better record of who’s attending than any other does. I never saw that growing up. (As an example I was still on the rolls for the Methodist church I was confirmed in in 1989 until the mid 2000s as an active member, even though I hadn’t attended a service there in about ten years.) I think this is as much by self-identification as it is by anything else - the map is from 2010, post-census, I’m sure, as most demographic maps of this nature seem to be. So this likely was drawn from census records and not church attendance records. I can’t say I ever saw someone taking roll as we walked in the sanctuary.