It wasn't for lack of need.
https://www.nytimes.com/2020/04/02/nyregion/ny-coronavirus-u...
> But the reality has been different. A tangle of military protocols and bureaucratic hurdles has prevented the Comfort from accepting many patients at all.
> On top of its strict rules preventing people infected with the virus from coming on board, the Navy is also refusing to treat a host of other conditions. Guidelines disseminated to hospitals included a list of 49 medical conditions that would exclude a patient from admittance to the ship.
> Ambulances cannot take patients directly to the Comfort; they must first deliver patients to a city hospital for a lengthy evaluation — including a test for the virus — and then pick them up again for transport to the ship.
The ships weren't for COVID patients, they were for non-COVID cases so that hospitals could focus on the overflow of COVID patients that they were better equipped to handle. Turns out that at the beginning of a global pandemic, not a lot people opted to go to the hospital even if they needed to because of fear of infection.