Definitive editions are timed for anniversaries

Age of Empires 1 Definitive - October 26, 2017 (20th anniversary)

Age of Empires 2 Definitive - September 30, 2019 (20th anniversary)

Age of Empires 3 Definitive - October 18, 2020 (15th anniversary)

Age of Mythology Definitive - October 30, 2022 (20th anniversary)

All of them will be aviable only in Microsoft Store, because they don't want to divide the online playerbase. It seems that Age of Empires 2 HD won't get any new patch, they abandoned Steam and the last one will turn off the light. They are working on the new projects already.