The most simple answer for GW doing this is that is how 40k works. 40k has strict factions separated by races and that has worked for years, and they just repeated that practice here. Makes no sense lore wise, but players have gotten over that with time. Shame really since Cities was such a cool and different faction, but will most likely continue down the road of being Imperial Guard 2.0. Glad that most websites and youtube/podcast called them out on this (expect 1), and hopefully GW