It's a good way to make a soft reboot of a series. The primary title is now Imperator, instead of EU-spinoff Rome. The scope of the game have also changed dramatically, making it no longer a spinoff game with regional focus, but a full fledged franschise starter encompassing several ancient centers of civilization. Thus less of a need for "Rome" to be the primary title.
Also, it lessens the risk of being sued for trademark and copyright infringement by Creative Assembly.
I wouldn't be surprised if they did the same with the inevitable CK3, having the title of that game be either "Crusader Kings 3: #subtitle goes here#" or " #new main title#: Crusader Kings", since the scope of that game series have also changed dramatically.