No reason has been given as to what happened or why he was released, and DC has not released a statement yet on Didio or their future plans. Didio has been co-publisher of DC alongside Jim Lee since 2010.
Didio joined DC in 2002 and was promoted to vice president and executive editor in 2004. During his tenure, he oversaw the maxi-series 52, the line-wide relaunch The New 52 in 2011, the subsequent reboot DC Universe Rebirth in 2016, the launch of Black Label and the shuttering of the Vertigo imprint. He has also written several comics for DC over the years, including Superboy, Phantom Stranger, OMAC and the current Metal Men miniseries, among others.
Creators and people from around the comics industry have reacted to the news on Twitter: