Because there is a lot of characters that CA could pick.
For reference I believe MoH brought 2 portraits for "Titled" generals (Huangfu Song and Xun Yu) and 2 portraits for new/untitled generals (DiaoChan and Lu Zhi).
We also sorta have Liu Chong who is a newcomer (but he's not apparently playable unless you have MoH - not sure if you can recruit him without it either).
-> Amongst the current Titled generals tied to the Sun faction we obviously have Lu Su and Cheng Pu but also Zhou Tai (he is used in the Gan Ning spawn event I believe ?).
-> We also have the "goonies of Liu Biao" like Huang Zu and Cai Mao. but both of these guys alongside Liu Yao could - I reckon - be the "paid" factions of the pack. (So getting portraits but not being part of the free update per se)
-> Amongst the "untitled" there is also a crowd, Qiao sisters at the forefront I suppose (since they are like Diao Chan and spawned via event yet have no model/art for now)
What's your opinion guys ?