Mother passed away due to covid! To?

Hello Doctors I would like to thank each and everyone of you for risking your life and saving individual life affected by covid19. Unfortunately my mother passed away due to covid on 18th May 2021. We had bed, we had oxygen, we had remedesiver and we even doctor's were present on time. All the care and steroids were given. Still, my mother could not survive. My mother was admitted on 12th May and she passed away on 18th May. When admitted her CT score was 9/20. I believe it was moderate case. I used to talk to her daily and there were no problems except her throat infection. Infact, my brother was there at the hospital only.  Doctor said she will be discharged in next 3 days. We were very happy!! On 15th her oxygen started dipping. So, doctor suggested to use Oxygen externally. It started with 1 litre and went till 15 litres. Her oxygen was varying too much. Finally on 18th doctor put her on ventilator. But, within half hour she had cardiac arrest. So, my only que is wat went wrong?