> But there were thousands of empty covid-specific beds open in that state. Why where they not sending patients to these field hospitals?

There is a somewhat similar situation in the UK, where hospitals are dealing with a lot of COVID patients, but the 'Nightingale' hospitals, our equivalent of these field hospitals, are mostly empty.

Something i have heard, but do not have a source for, is that the Nightingale hospitals were built on the assumption that COVID patients just needed a bed and a ventilator, but it turns out that COVID causes multiple organ failure, treating which needs a wider range of facilities, and can progress very quickly from respiratory symptoms to that, so they aren't actually much use.