I have no inside information. I don't know anyone at Open AI. This is all purely speculation.
Now that that's out out the way, here is my guess: money.
These people never joined OpenAI to "advance sciences and arts" or to "change the world". They joined OpenAI to earn money. They think they can make more money with Sam Altman in charge.
Once again, this is completely all speculation. I have not spoken to anyone at Open AI or anyone at Microsoft or anyone at all really.