How was Solarwinds hacked and created a big mess?

Solarwinds hack has put 18000 organizations including US Nuclear security administration into a sinking boat. We are in total disaster. As per Microsoft, 44% IT companies are infected. Many organizations are still in discover mode and they do not know if they have hacked or not.

Major List of Organizations including Banks in India:

You would be surprise to know that Solarwinds update service was used because of their awesome password.

How did every security software miss this?