Empire are from the first game so they quite easily could have ignored them..
Of course they couldn't ignore them, seriously, think about it logically). The Empire are pivotal to the 3rd Game as an arch-enemy of Chaos and they are overall the most important race in the entire Warhammer world. It wasn't a question of
if they would be updated, but
when. Otherwise do you really think they would update Bretonnia, Dwarves and Undead, both Game 1 races? Orcs will likely get the update next and all Game1 races will ultimately will receive some kind of update or rework (varying in relative effort I am sure, with Beastmen getting the least) to bring them to at least a reasonably competitive playing field with the Game 2 and upcoming races Game 3 races.
The Mortal Empires map with all races added is the ultimate future and longevity of this series and it has to be designed with that in mind.