They quit because the new Manhattan District Attorney does not support their investigation. The new DA is a BLM-type DA, so if he doesn't support it, it must mean that they've come up empty. There is no there there.
“...the new Manhattan District Attorney does not support their investigation. The new DA is a BLM-type DA...”
And the old DA was Cyrus Vance Jr, who was after Trump. He left on New Year’s Eve.