clinicaltrials.gov - MSCs in COVID-19 ARDS, page-3

It's too complicated to have the trial including people who do not require ventilators. The survival rate is much higher, meaning more participants needed to prove significance, and they would be wasting a lot of good cells on people who were never going to die anyway.

Save the cells for those who actually need them, and go for the conditions with the highest death rate to show as fast as possible and without a hint of doubt, that the treatment works to stop people from dying.

What's the point of dosing people who were never going to die, that's not the point of this treatment, we will not have enough cells to give to everyone, only those who are dying.