tbh I can speak from Luna.. when we found out she was blind I had so many people ask me if I'd rehome her or even PTS because she's disabled..? & then when she got diagnosed with arthritis at 2.6 years old roughly. I got asked if I'd rehome her or what Id do since the metacam could kill her. I mean..sure.. but I wanna make her comfortable ? even if that knocks some years off of her. idek
& yeah
they decided he wasn't easily adoptable due to his megacolon. being a rescue, they can easily have in IV fluids, medicine, always at hand. for somebody else adopting, it might prove difficult for them to have meds on hand. it may have to wait until they get a vet appt, which may be too late. & then finding somebody comfortable to even do IV fluids at home.. they decided he has the best care at the rescue (well, he lives in her house for constant monitoring) and didn't feel comfortable adopting him out.